"nothing is fair" BUT he has the internet, rules out north korea lol
stay away from matrix, they are total crap
cyma and king arms might be your best bet
.20 and .25 are the weight of the ball, the heavier the ball the further you can shoot with it.
back wired would be best for your build, since you might be taking the gun apart a lot to fix things.
The full M16 stock would be easiest to use, but you can get a 7.4v 1200mah lipo battery that fits in the stock tube, so really any retractable stock will work with rear wired.