I would recommend a normal AR with a 5.56 chamber or a Wylde Chamber so you can shoot both 5.56 and .223 ammo and not have to worry of over pressures from the 5.56.

The normal AR will be cheaper then a pistol caliber upper.

The ammo is relativly cheap and in some cases cheaper then pistol ammo. You can get 1k rounds of Wolf for $189 thru online stores like Aim Surplus. There are also deals from private sellers all the time on AR15.com Equipment Exchange or even on here, just gotta be quick if its on this wonderful site.

Good Luck and post pics of your first AR!!!