If I only had $700, I'd buy a AKM clone, probably from Vector, and spend the remainder on mags and a case of 7.62x39. AR-15's are not a 'budget' rifle. I firmly believe that people who want a serious weapon are better off not buying a cheap AR-15.
IMHO it's also pretty silly when someone who "can't afford a more expensive AR", chrome lining, etc., ends up with 2 or 3 cheap rifles each with 3 pounds of generic tactical crap, cheap lasers, and $30 chinese red dots on them.
Oly and DPMS aren't crap, really, but they just don't seem to have the track record of the other AR makers, and there are better deals out there. Also, for your first AR, at least get a factory complete lower and upper, don't build your own yet.
Your best bet for a nice but cost effective setup is pretty much what Salmonaxe said, CMT/Stag makes top quality stuff.