PSA or Palmetto State Armory
has great pricing on quality AR's. They are mail order only if not local. If you are going to buy local then the S&W is a nice AR for the money.
The 5.56 and .223 are essentially the same physical size. The 5.56 round well normally run your AR better than the .223 ammo, but you can shoot either one.
Keep the nitro pills close and have fun.