Comments:
BMW's flagship coupes from the 80s and 90s were really special with styling that wasn't controversial. Easy to like!
Biggest problem with the 840 & 850 is Big Ownership Cost
I know we are just dream-talking about the 8 Series, which I have also always loved. But when I looked intoi buying one, I got scared off.
One of the trade magazines I read at the time the cars came on the market said they were the most expensive car to own and run in America - a combination of parts and labor costs multiplied by electrical systems unreliability (that was when new!) So you can imagine the running costs today...
Here's some info from Driver Advisor (edited for brevity)
Are BMW Less Reliable Than Other Car Brands?
Is a BMW unreliable? We went to repairpal.com which has data from all different car brands since they run numerous repair shops around the country. We gathered information on 26 car brands sold in the United States. We selected the three most important categories for all these brands: the reliability rating, the number of unexpected visits to a repair shop annually, and the probability of the car having a severe problem when that unexpected visit is made. The results are displayed in the table below and are sorted by reliability rating. Scroll all the way down to the bottom of the table to find the results for BMW.
Brand Reliability Shop Visits Severe Problem Probability
Honda 4 0.31 10%
Toyota 4 0.34 12%
~~~
M-B 3 0.73 13%
Audi 3 0.8 13%
BMW 2.5 0.9 15%
Out of all the brands sold in the United States, we found that BMW has the lowest reliability rating of 2.5 out of 5. BMWs show up in RepairPal shops disproportionally often and problems are worse than average, and also cost more. BMWs, on average, have to go to a repair shop for an unexpected visit around 0.9 times per year (this is the highest score of all brands, which is not a good thing). This means you can expect a non-planned stop at a repair shop almost annually. When doing so, there’s a 15% chance of your BMW having a severe problem (higher than many other brands}.
How Many Miles Can BMW please the owner?
So, BMWs may be unreliable. Reliability also indicates the potential lifespan of a car. In other words, after how many miles can a BMW's owner no longer tolerate this?
We went to autotrader.com and selected all used BMWs in a particular series that were for sale in the United States. This usually gave us several thousand cars in a group. Then, we divided all the available cars up into groups based on the mileage they had driven. This helped us understand what mileage you could expect from a specific model of BMW and how many reached that mileage before the owner put them up for sale. Again, got to the site and read the full article.
Model Ave. Mileage at sale
5 Series 250,000
3 Series 240,000
M3 230,000
X5 220,000
X3 210,000
7 Series 200,000
M5 200,000
Z4 200,000
Z3 180,000
X1 170,000
M4 170,000
X6 165,000
4 Series 140,000
8 Series 130,000
X4 130,000
2 Series 110,000
It seems that 4-series, 8-series, X4, and the 2-series have a shorter ownership/lifespan. This may indicate that there’s something wrong with the cars or their owners are fickle. With an 8 – 10 years lifespan, they run out of life energy (owner love) quite quickly.
Do BMWs Cost Much In Maintenance (in the USA)?
In this blog post, we did extensive research on the maintenance cost of BMW. Average annual maintenance cost is another critical factor that in establishing whether or not a BMW is desirable or not. We went to repairpal.com again and took the 26 most popular brands, for the average, cheapest and most expensive maintenance.
Brand Annual Maintenance Costs
Honda $428
Toyota $441
Mazda $462
Hyundai $468
Kia $474
Nissan $500
Acura $501
Mitsubisi $535
Lexus $551
Buick $608
Subaru $617
Dodge $634
Jeep $634
Infiniti $638
Chevrolet $649
Volks $676
GMC $744
Volvo $769
Ford $775
Cadillac $783
RAM $858
Lincoln $879
Mercedes $908
BMW $968
I'm not trying to diminish the appeal of a BMW to the average buyer and driver. But from a repair shop's perspective, you couldn't ask for a better revenue source than a customer with an 8 Series BMW.
I went to Edmunds (a former client of mine) where they create what they call True Cost of Ownership. Since the current site only shows back to 2017 I had to select a 7 Series V12 as the sample car:
Here are their numbers:
Ownership cost is something which frightened me with all BMWs, I must say. What an answer, Mike!
Maybe I just had good luck but my 850 was pretty reasonable.
The car never left me stranded, no drivetrain work needed, just little things like headlight motors, window seals, a/c recharge, etc. Sometimes I think journalists at the time were just annoyed that it was like owning two BMWs, 2 batteries to replace, two sets of plugs and injectors, two mafs, etc but when it came down to it these were fairly simple and relatively unstressed engines. The real scare was getting a nikasil v8 in the early 840s, then you might be looking at a block replacement. Otherwise, not too many huge issues.
Oh you took us back in the memory lane ..
V12 is the way to go & remains on my grail list … Alpina and M versions were amazing
Yes, memory lane... And a delicious one. ;) As for the Alpina, I was lucky to test drive a 3 series from the late 70's, based on the 323 I. It was some wild and fun to drive!
Unappreciated for it's time but I miss it now, put many miles on mine including a cross country trip from LA to NYC
Cruising at triple digits speeds was unlike anything else in the 850, the v12 was ever present but so smooth and would hum away on the freeway. Cabin ergonomics were some of the best in nay car I've every driven, the b-pillarless design plus a sunroof made the car a fun and airy drive weekend driver with a great greenhouse despite the low roofline, and the 6 speed made it silly car in first and second with great torque off the line. I used to sometimes start in 3rd if I was feeling lazy or it was snowing out. Yup, drove it year round on summer tires. By today's standards it's a small car, when the new 6 series came out I remember pulling up next to one and being shocked to find the top of the door was almost at eye level. Compared to the epas systems today, the 8 had way better steering feel, and 4k lbs isn't really a heavy car anymore. It's modern enough it has most of the conveniences we need and 300 ish hp with a manual is plenty for a weekend car. My car had custom sway bars with welded subframe reinforcements, stiffer springs/shocks, poly bushings all around, enlarged intake manifolds, shorter final drive LSD from a 750, and the old school ecu tune that required swapping eproms 
At the time I think the 850 was a failure because it was so much bigger than the cars the came before it. Next to my E28 which seated 4 comfortably, the 850 was gigantic, soft, and heavy while feeling tiny inside. The automatics were TERRIBLE and totally masked the character of the v12 and while it was fastish, the design made it look faster than it was. If it has been designed to look like a bentley no problem but since it looked more like a ferrari, perceived performance was a little disappointing unless you got the CSI which even then was probably outclassed by the 3.8L E34M5, only 340hp an down on torque but a more aggressive sporty engine in a lighter chassis and quicker shifting 6 speed. Today though, taken on it's own instead of relative to the pack at the time, it's such a unique and pleasant driving experience that I would love to have one if space allowed. Maintenance was not as bad as the media made them out to be, just electronics here and there to be replaced and of course you have to buy two of everything over the previous BMW i6 models, but the engine and transmission were overbuilt so mechanically these things were tanks.
At the time I think the 850 was a failure because it was so much bigger than the cars the came before it. Next to my E28 which seated 4 comfortably, the 850 was gigantic, soft, and heavy while feeling tiny inside. The automatics were TERRIBLE and totally masked the character of the v12 and while it was fastish, the design made it look faster than it was. If it has been designed to look like a bentley no problem but since it looked more like a ferrari, perceived performance was a little disappointing unless you got the CSI which even then was probably outclassed by the 3.8L E34M5, only 340hp an down on torque but a more aggressive sporty engine in a lighter chassis and quicker shifting 6 speed. Today though, taken on it's own instead of relative to the pack at the time, it's such a unique and pleasant driving experience that I would love to have one if space allowed. Maintenance was not as bad as the media made them out to be, just electronics here and there to be replaced and of course you have to buy two of everything over the previous BMW i6 models, but the engine and transmission were overbuilt so mechanically these things were tanks.
'88 M5, at the time the E30 M3 was too expensive! I bought my M5 for $9k, couldn't find an M3 for less than $15k!!! :P
Became my auto-x and track car since I figured at the time E28 5 series panels and parts would be easy to replace if I had an incident. Who knew those would become $100k cars 
I swapped in some race seats, harnesses, bigger brakes, straight pipes, this car was insane. Went slightly too big on the rear brakes which could made the car a hooligan on track, the car just wanted to rotate on the brakes all the time making it easy to get the car into turns mostly for the better but you had to stay on your toes. Where the 850 might have been a little porky for its era, the M5 was an angry little car that looked like a family sedan. It was so good I've contemplated selling my Cayman to have one of these as an everyday driver again but can't bring myself to pay 10x what I sold mine for.
I swapped in some race seats, harnesses, bigger brakes, straight pipes, this car was insane. Went slightly too big on the rear brakes which could made the car a hooligan on track, the car just wanted to rotate on the brakes all the time making it easy to get the car into turns mostly for the better but you had to stay on your toes. Where the 850 might have been a little porky for its era, the M5 was an angry little car that looked like a family sedan. It was so good I've contemplated selling my Cayman to have one of these as an everyday driver again but can't bring myself to pay 10x what I sold mine for.
If you ever decide to go for an 840/850 I think it would be a fun weekend car! Though I imagine it would be like driving two A110s on top of each other in a turn :)
I know you mentioned leaning towards the V8 and it certainly has it's advantages and the numbers are close enough but I hope you are table to test drive both, there's something about the feel of the v12 that's worth experiencing too. The sound of the huge starter to crank the big engine, the exhaust noise of course, and the smooth but still mechanical feel of the engine. There aren't that many opportunities for a manual v12 for reasonable money and it's an experience even if not really any more powerful or faster than the v8.
In my younger days,
I presented my little brother with the red model, as a model though.
Little time to get his degree in architecture.
And no car as of yet. But he enjoys 1990's B.M.W., so who knows?