Hello Mr 303
As members here have suggested the 3570.50 will be a very nice watch to have and in terms of buying new, as close to the original as you can get.
I love my 3573.50 because I like the clarity of the sapphire and the display back allows me to view the lovely, decorated rhodium-plated cal 1863.
However here are some alternatives from the pre-loved market -
3572.50
which is hesalite-fronted but has a display back in sapphire so you can view the movement. Earlier models ( 3592.50 ) have a YG-plated cal 863 movement while later models use the Cal 1863 that is Rhodium-plated.
3570.50 as mentioned
145.022
which is pre 3570.50. Some may still be found and pre-92 models have the cal 861 in pink-gold following which they were changed to YG plating. Earlier models also feature the S and r of the Speedmaster legend slightly elongated. Just makes it a little more desirable
Early cal321 models but you'll need further advice than what I can manage on these.
Limited Editions
Again all pre-loved and the ones you are more likely to see/find are the -
Apollo XVII mentioned by Chrisfrombyron,
On occasion, the Apollo XI 30th Anniversary with the " Hello Houston....." caseback
Some models from the Missions Series which will have the mission patch on the continuous seconds counter
Very desirable - the Snoopy Edition
Hunt around and you may still find the part RG-plated Apollo XV LE from dealers
Remember that the values of Speedies on the pre-loved market has increased markedly over the past twelve months, from 25 to nearly 50%. A year ago you could find a pre-loved 3570.50 for around $1250. Now they can go anywhere from $1650 to $1850.
WISh you well in your hunt
Houston