where do i start? i can find the parts, the amo, the mags, but what do i want to build? i want a fire arm that can be upgraded and changed. this is a build and not a purchase, i dont want to buy the gun whole. reason follows. i make a finite amount of money, i can buy the receiver this month and the parts kit the next. i might be able to buy the upper over a couple of months but i have bills to pay as well as a wife and kids to feed. i cant afforrd to spend the money on a firearm like this all at once. where do i start and what do i start with? thanks you for your time!