If you want a "Mil spec" upper, look into the LMT or the Bravo Company offerings. You will pay more, but they are better quality parts (ie magnetic particle tested, 1/7 twist, ect).
You need to also consider a few things.
What type of barrel (HBAR, Med-contour, light-weight)?
What type of gas system (carbine or mid length)?
Personally I don't mind a heavy barrel. I have a RRA 16" in HBAR with a mid-length gas system and I love that thing. I run a RRA 9mm buffer (heavier) and I have very little trouble with follow up shots. RRA also has a two stage trigger.
I also have a STAG 16" with a gov-profile M4gery (N203 cuts) barrel and a carbine gas system. I also run a 9mm buffer in it and it follows up quick, but not as quick as the middy.
Both of my rifles are parst guns, but I used RRA upper/lower and STAG upper/lower parts.
Spend your cash on a good stock. I have a CTM from magpul and I recommend it. Not the MOE stock, but the actual CTM with friction lock.
I'd also say find a grip that suites you well. The MOE from magpul is a great bargin as long as you feel comfortable with the slightly thicker grip (I do).
Get some Break free CLP. The AR likes to be lubed. Keep it lubed and it will serve you well.
Save your cash and skip the fore rail up front. Spend that on ammo. The AR is a blast to shoot and the more you shoot the more comfortable and effective you will be.