i would get the saiga .223 unless you plan to do a total convert front handguard retain etc
all i changed on mine was the buttstock , triger group, pistol grip, gas piston and front sight base
saiga .223 350$ slr106fr 800$
if i didnt have other .223s i would get a 5.45 SGL31
i have 2 ar15s and didnt want to add another caliber
i have shot 5.45 and it shoots as well as 5.56 plus it feeds better in an AK