You should definitely get a flat top and make sure you have the Armalite match trigger. free float handguard is a plus. for about 300 yards, a match grade Armalite 20 inch barrel will be fine. Your only other consideration is a scope and rings. Leupold makes one of the best scope in the world, and its price is set to the size of your wallet. Nightforce also makes a great scope. the price is about three hundred dollars higher then a same comparable model Leupold, but I think nightforce is a better and more durable scope. You will want a heavy duty rings. Leupold Mark4 rings, GG&G, Badger and ARMS are excellent rings. If you are looking for quick detachment, ARMS is the way to go. If not, you should stick to something like a GG&G steel ring for a security mounting solution. make sure you get some 20 rds mags, is is a lot better to shoot on a bench or prone.
Oh, almost forgot, you might want a bi-pod. some people don't like them, but buying a Harris not wallet breaking like a Parker hale. i like the Harris that can swivel from side to side. gives me a bit wider range of adjustment.
There are other company besides Arma, and bush. DPMS, or Knights armament is two other comany that comes to mind.