There are 223 AKs, but the mags and spare parts are not as plentiful and more $$.
For 223, if you must, AR15 is the best platform, a 16" barrel, flat top, flat gas block, collapsable stock, is the route to go. Bushmaster is a great flavor, Rock River Arms is nice as well. Several good makers, DPMS, Stag. I would steer clear of Olympic, though some would say they are fine.
You could build, but I only suggest that if you already have a quality rifle for SHTF, other wise you may find yourself with an incomplete rifle when the turd gets to the fan.
I would suggest that you consider the M1a Standard. It will give you more utility, less specialized, quality rifle able to reach out to 500-600 yards and be effective when it gets there. FALs are my second choice, and are nearly as capable.
Mags and ammo, mags and ammo, mags and ammo, mags and ammo.......and one last thing, MAGS AND AMMO.
AR mags= $10 FAL mags= $10 M1a mags= $24
Ammo= good luck!