At the risk of sounding like I own stock in the company, may I suggest you check out the Smith & Wesson M&P 15-22. It is a dedicated system (not a conventional rifle tarted up with MIL-SPEC looking furniture) that, while a blowback-operated rifle, has remarkable similarity to .223-and-above AR-type firearms. Almost anything that can be attached to an AR (lights, sights, grips, slings, etc.) can be used on a 15-22 and there are many owners who have decided to put accessories worth double the cost of the rifle on the things. The best part about that is that what fits the 15-22 also fits the larger-caliber guns. Even the fire control assembly is AR-standard and can be modified just like an AR. There is also an active community of 15-22 shooters online with all the advice (good and not-so-good) and information you will ever need. A dedicated upper for your AR is expensive and they do seem to have more reliability problems than they should. Your money, your choice; but you owe it to yourself to take a look at the M&P 15-22. Good luck.
"History teaches that war begins when nations believe that the price of aggression is cheap." - Ronald Reagan