Which mags are you thinking about using??? Have you seen any complete uppers for a decent price while looking around different sites.
The C-Products or Stoner 10rd mags to start. Still looking for cheaper ammo - Federal was supposed to be producing some but I haven't seen it.
The cheapest complete uppers I've seen were $800+ I'm not going to save a ton going piece by piece - but I'll be getting the pieces/look that I want.