They've all been mentioned but I'll list again in no particular order:
1) Colt 6920
2) BCM
3) Daniel Defense
All are top tier ARs and well within your budget. I personally own the Colt.
Re building versus buying for a first AR, I recommend buying a complete rifle from above list that'll run like a champ right out of the box. As a new AR shooter you'll then get to learn how its supposed to work, what you like and dislike. THEN go build whatever you want 'cuz you'll want another!!! Or at least some mods...
Another amazingly educational site:
http://www.m4carbine.net/forum.php