You can get a LMT upper minus the bolt group and charging handle for $485 from:
BRAVO COMPANY USAYou can then buy a RRA enhanced bolt group for $95 and a RRA charing handle for $16 at
EAGLE FIREARMS . Total cost would be $595 plus S&H. Seems to be a little out of your price range, but LMT is top notch quality.
LMT M4 upper can be had in 10.5", 14.5", or 16" lengths (all the same price), and come with an A2 FH
Bravo Company all includes M4 hand guards for free. The LMT upper has 1/7 twist with chrome lined barrel made of 4150 steel, not the lesser quality 4140 steel. It also has M4 feed ramps, which increase the reliability of the gun.
U should also check out
G and R Tactical. They also sell LMT uppers, and are having a special deal on an LMT upper with LaRue 7.0 Free Float tube for $712, minus bolt group and charging handle.
Edit: I found another post here on the forums...
RB PRECISION is selling an RRA "enhanced" bolt group for $99 shipped.