I've not seen an AR yet that I'd be happy with for $650 or under.

There are plenty of them that can be bought for cheap but that's what you're buying, cheap. You'll wind up replacing the trigger first, then the barrel, maybe a hand guard, furniture and such. Look around and see what you want then buy it, don't limit yourself. You could do worse than looking at a LaRue kit.
https://www.larue.com/products/larue-ultimate-ar-15-upper-kit/

Now there will be plenty of souls along to tell you their best buy is all that and a bag of chips, be careful who you listen to...