Here we go again . . . and this time a 2-fer!

ARs - any of the well-known brands are excellent. In all the time this site has been operational I have not heard one statement here along the lines of "Do not buy this AR because...." You
will hear those statements on other sites which leads to the confusion. Trust the folks here, they have proven their worth and knowledge without the strident emotions evident elsewhere. So, DPMS, Colt, Stag, RRA, Bushmaster, etc. are excellent. Spikes Tactical also is alleged to do a good one, although I do not know anyone who owns one - other than MB. Remington has their new R-15 too (Remington is owned by the same company that also owns Bushmaster).
223 vs 5.56 - buy a rifle marked for 5.56 and you can shoot 223, but not the other way around. Has to do with different SAAMI pressures and case lengths.
Welcome aboard to the site.