Got 3 AR's, a Colt, a Bushmaster and a DMPS or something like that, cant remember the initials. The way I have it understood is all the parts pretty much come from the same suppliers across the board. I'm sure some go the extra mile to add whatever to make theirs special.
After seeing Marines take old worn out Colts on the range that had tens of thousands of rounds fired through them, and be able to put 10 rounds in a 6 inch hole from 500 yards away made me realize that the brand really isn't the question. Its an issue of weather or not you can shoot. You can buy all the big names you want, but if you cant hit the broad side of a barn, no brandname is going to change that for you.
If its something you want to sell later down the road, better get a good name brand then. But if its something you are going to keep in your closet and fire for fun to improve your marksmanship, then it doesn't matter much whose name is on the upper/lower receiver.
If the AR is clean and well taken care of, $600 is a far price in my book. Have seen used ones go for less, and have seen used ones go for more.