I've been looking at trying to pull together some $$ and assemble an AR by buying a complete lower and a complete upper separately. My budget is going to be about $500 for the rifle (won't be able to afford much ammo or many mags for awhile but I'd like to at least get the rifle). How likely is it that I could piece together an AR with a 6 position stock and a 16" A2 or A3 upper for that amount? What about a full length upper? I am planning to probably go with better names - probably Stag, Bushmaster, or RRA and I will be shopping strictly used. Can this be done or am I living in fantasy land?