Lots of different caliber uppers will fit but in general, ammo wise, they all cost a lot more than .223, with the exception of 5.45x39. I'd stick with 5.56/.223.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together. You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek