Congrats on your new armor and good luck with the build. It's a great feeling building it.
I can't really help you with your first question. I used some snaps I picked up at Hobby Lobby....Your number 2 question, I don't think it'd be a problem with using elastics like you mentioned. The most important thing is the outside look of the armor. If you go for EIB and/or Centurion, then the gap between your ab and kidney plate will come into play. #3, I used testors flat black paint. I used light greay primer first and then applied the black paint and it came out fine...
The guys who've been doing this longer will give you more probably but hope that helps some...