Remember to think about caliber, there are AK variants in a variety of cartridges.
Generally a 7.62*39 or 5.56*45 (same as AR) gun will serve you best. 5.45x39 is a great caliber but the ammo gets a little scarce sometimes but still cheaper than 5.56 when it comes in. Id recommend it only for a 2nd AK.
If you are tempted into an underfolder, remember they are very uncomfortable guns to shoot. Generally a fixed stock or the plastic sidefolder on the Arsenal SLR series will be most comfortable.