also 7.62 AK47 vs 5.45 AK74
Past that i'm not familiar on any true operational difference. Typically your 74 is more compact... or at least the one's i've seen. That's not a necessity of operation, just what i've noticed. All the AK's that come after 47 are mostly to accomodate a caliber change or a modification in the mechanical functioning of the rifle. AK101 for example is a 5.56, while the AKM is a 7.62
ETA: I can't speak 100% on 922r compliance, but it has to do with the importation of firearms and what can and can't come across the border. As i understand it, a fully assembled AK, can't legally travel into the US borders anymore, for that matter, no pistol gripped/flash suppressed/grenade launcher equipped weapon can. But you can import everything but those parts and then buy them here since buying them in the US is legal, just not importing them (don't try to make logic out of gun laws, they're illegal in the first place so what's the point).
In my experience. Just buy an already compliant rifle, at least for your first time. Learn from there.
ETA2: http://www.classicarms.us is a good place if you're just starting to look around.