just to address your worries about having problems finding ammo: you won't have problems either way. A 5.56 chamber will fire .223 no problem. But you want to avoid firing 5.56 in a .223 chamber. Since 5.56 is less common than .223 (at say Wal Mart or some gun store), and .223 will work in a chamber cut either way, you will have NO problems finding ammo you can use. But of course, a .223 chamber is limited to .223 ammo.
One other thing- I know it's blasphemy to say this here, but put off getting your AR. Do your homework. You're new to this and there's a lot to learn. This site can be a valuable resource, but don't use it as your only source of learning. You'll occasionally run into misinformation here that can lead you astray. And you'll sometimes see some "Texas Truths" (if more than 3 people say it then it must be true). Furthermore, (pointing out the obvious) the average person here is, of course, an average AR-15 shooter. And you don't want to learn about a weapon system from people who are only average in knowledge and expertise. This isn't to say that everyone here is a dumb ass. This site, and an awful lot of it's members are helpfull and knowledgeable in many ways. But why base your knowledge on what we (questionable sources) say when you can buy books on the AR-15 written by authors who are so credible that they actually get PUBLISHED. In the long run, you'll be better served taking your time, putting out the effort, and doing it right.
Disclaimer: I have nothing against Texas. Of the 2 times I've been there, I enjoyed myself twice.