Right now almost all brands of AR are "OK", though there are major quality differences. Below $1k you aren't getting premium quality, though S&W is sometimes/often available in that range and would be a good choice. I have found Bushmaster to be fine - cosmetically so-so and they cut a few corners, but overall pretty good. Armalite is cosmetically nicer and more refined. I recently assembled an AR lower using a DPMS parts kit, and the experience, plus buying and returning a DPMS rear sight (BUIS) due to the way it was made, has me NOT recommending DPMS if you have other options. I don't own a Rock River and reports vary from terrific to just fair. CMMG is pretty solid, though not a standout. All in all I'd try to find a S&W, Stag, or Armalite in your price range, but I would be happy with a Bushmaster, CMMG or Rock River if I couldn't find the first three in my price range.
If a couple $100 more could be found, I would consider LMT, Colt, and Sabre Defense as good options also.
The A3/A4 (same thing in civilian world) flattops are more versatile but do add to your cost a bit if you're only going to use iron sights. The A2 setup will limit you but is also simpler, lighter, and usually cheaper if you're sure you're only going to be using the iron sights. If you get a flattop, buy a quality rear sight or carry handle, not one of the imported cheapies. US-made carry handles can be found in the $70-80 range with some searching - I recently saw one listed at Centerfire Systems (but beware that they're also selling a Chinese copy for $30; it won't be the same thing!).
Be sure to leave enough money for a bunch of magazines, ammo, and the usual cleaning and shooting supplies.