Tons of information on Ar's, too much to go into detail here.
I'll give a few bullet points to get you headed in the right direction.
Get one that is a 'flat top', meaning one with out an attached carry handle.
Triggers and barrels get a lot of the $ invested in a good build. 18"-20" barrel with 8 twist medium contour,SS with threaded muzzle.
Direct impingement over gas piston, unless you are going to run a suppressor full time. Free float hand guard with a way to attach a bi-pod. Unless you want the Rambo look, no real need for a sh!t load of rail/ attachment points.
Rock River makes a great 2-stage match trigger that will serve very well for about $125.00. Or a Jewell for about $225.00
Seekins has some of, if not the best ,AR parts out there.
http://www.seekinsprecision.com/Most upper and lower receivers from reputable manufactures will work great for a build.