Get a flat top with a removable carry handle and a railed gas block with a flip up front sight, that way when you do put a scope or optical sight on it (and you will as your eyesite fades), the front sight does not ghost in the scope.
Stick with the 556 chamber for simplicity, as the wylde chamber isn't that common on low to mid end ARs. 55 grains will still shoot fine in a 1/7 twist, but lighter bullets will tend to overstabilize in it. You will get everything you need in a 16 inch carbine versus a 20 inch rifle. The current M4 is a 16 inch barrel, but the 18 inch has a lot going for it, too.
Dan is right on the target rifles, with 2 stage triggers being great for targets.
I like a 16 inch fluted barrel, just for weight. One of my Wilson Combats is a UT15, has the 16 inch fluted barrel and a JP single stage trigger set at three pounds. The other Wilson is the m4 version, barrel is heavier and not guaranteed to be as accurate.
Do not buy a Vulcan Arms, Olympic or Hesse. I do not recommend DPMS either, but some will. I recommend Armalite, Bushmaster, Colt, YHM, Smith and Wesson, Wilson Combat, Les Baer, JP Enterprises, Rock River, Larue Tactical, Stoner, LWRS, POF (Patriot Ordinance Factory), and LMT (Lewis Machine and Tool). I am sure I have missed some that someone else will be along to chime in on. A major difference in them is how well they were finished.
You can do just about anything you want, how much do you want to spend? There are dozens, if not in the hundreds, of AR15 manufacturers, but only a handful of places that makes lowers, and only a few more that actually make the uppers. Most uppers are proof marked, and an internet search on ar15.com will tell you where they were made. The upper and lower blanks are sold to the companies to finish them and engrave their names and serial numbers.
Find one slightly used, and save your money for all the upgrades, like a Hogue grip, Larue Tactical, Knights Armament, Surefire, YHM or Daniel Defense quad rail, A gas buster charging handle, LMT Sopmod or Magpul collapsible stock and an A2 buffer.
My God, why did you get me started?
I do have a Colt 6920 for sale, 16 inch m4 profile barrel, shot around 200 times. $1400 is the price.