1. Which one is more collectible or easier to sell in the future if somehing happen to me
2. Is it true that Patek 5146 now has Caliber 324? Patek website is still showing Caliber 315
3. Would you recommend me to buy pre-owned at ebay for far less money than brand new at local AD?
Your input are appreciated
1. Which one is more collectible or easier to sell in the future if somehing happen to me
a, all about the same, there's fans of all 3
2. Is it true that Patek 5146 now has Caliber 324? Patek website is still showing Caliber 315
a. Eventually 324 will replace 315
3. Would you recommend me to buy pre-owned at ebay for far less money than brand new at local AD?
a. maybe, maybe not

1. I prefer the 5146 hands down, based on appearance alone. (I know nothing of the insides and workings but I have to love the appearance/look first above anything else before I start thinking of buying. The one that I like the best might not be the one you like best, so choose the one that appeals to your own taste. You'll know which one. Try it and and the watch will just feel right and smile at you.
I am waiting for the 5146 in rose gold. (In fact, I called the PP AD just today and asked about when it will get here. I could arrive here around Sep (but that's with a maybe still.)
As far as which is more collectable, well, ask in about 20 years, please; by then we might know LOL! I wish I knew the answer now to tell you and I'd sell the house to buy many. But all PPs in general tend to maintain high resale values, so if anything your loss would be less than owning some other brands. And if inflation, price increases, time, and collectability all factor in your favor, you could, with a big maybe, come out ahead.
2. As said, the 315 is being replaced by the 324. (Now, my question to the members is if the watches with the 315's are selling for less than those with the 324s?)
3) I have bought mostly through gray market dealers (not official ADs), however, I have bought all my through reputable greay market dealers who have owned bricks-and-mortar stores for decades where I can actually go in and try the watch and see it in person. I also live in a place where gray market dealers and their used watches are sold under the highest business standards. I am not sure if this situation applies where you live, so you'll have to decide that for youself. Neverhtless, as mentioned, high-end watches and sight-unseen auction sites at times don't mix. You have to really know more about the seller than the watch.
Good luck in your search. I am sure there is a beautiful PP out there for you.