In regard to ammunition prices, you'd be better off getting a 1:9 Twist barrel that wil handle the 55 grain ammunition since it is readily available. Try to get your hands on a chrome lined, cold hammer forged 1:7 twist barrel now while the prices are a little lower so that when the 62 grain .556 NATO rounds become a little more availble, you'll be equipped to swap out for a more powerful setup.
Rock River, Lewis Machine & Tool, and Daniel Defense are the brands I recommend.
**Disclaimer, I work for Daniel Defense in Savannah***
If budget is your main consideration, a DPMS or Bushmaster will serve you fine for now.
FYI, if you want some info on the AR/M4 platform, go to
www.ddaction.com
Thanks for your service.