Read this, it has all the info you need:
So you want to buy an AR-15, huh? - Police Forums & Law Enforcement Forums @ Officer.com
I would be sure to get one with an A3 upper (to mount optics, or even a carry handle if you want one), 1/7 or 1/9 twist 5.56 barrel, milspec buffer tube and firing pins (so you can use standard parts).
I own a LMT Defender in M-4 style and I love it. You can expect to pay $1200 or so for one new.