i wouldn't use a MINI 30 as your go to rifle, because most of the cheap 7.62 rounds use corrosive primer, that means you probably need to clean it everytime you fire the weapon. it does have better knock down power than the MINI 14. end of world rifle, i'd chose PCC , alot of knock down power, accurate at 100 yard.
Depends on your definition of "cheap". Commercial 7.62x39mm is dropping back into the $200/1000 realm again.
Pistol cal carbines and stopping power? Completely different league than a rifle-cal carbine. Pistol calibers don't generate the velocity needed to do the kind of damage you'll get out of a rifle caliber.
Just a note - I'd include .30 Carbine with the pistol calibers. It's at or just below that magic 2000 FPS mark when fired from a carbine.