Hi,
1) The 5396 is my favorite so the more collectible for me. I really like 5135 too but that's a very large watch. you need the wrist for it, I don't have it. The 5146 is a great piece too but as I prefer the 5396 I can't prefer the 5146! 
2) Calibre 315 is progressively being replaced by the calibre 324. As all watches are not sold immediately when reaching the stores, you may find watches with either calibre in the market.
3) No, but that's your money, not mine!
Cheers
Dje