If you want a bambi blaster, the 7.62x39 is not a bad choice, but why bother with the AR platform. You might as well pick up a Saiga, it's cheaper than an AR upper.
If you are going to spend the money on an AR upper, why not a .50 Beowulf upper which will drop bambi in her tracks. Certainly 7.62x39 will kill deer, that's not a problem although the stopping power is certainly less than what a 400 grain .50 Beowulf will put on the target.
So I guess, I don't understand why the AR upper in 7.62x39 just for hunting. Now if you have a bunch of other 7.62x39 rifles and a bunch of cheap ammo, perhaps. However as you said, it's just a lot of money to drop on an upper that will marginally outperform the .223 on deer (~30% more energy at 200 yards.) By contrast at 200 yards, the beowulf is capable of putting 200% more energy into the target and offering more bang for the buck.... pardon the really bad pun.
Energy at 200 yards
.223 648 ft/lbs
7.62x39 882 ft/lbs
.50 Beowulf 1417 ft/lbs
Now the downside to the Beowulf, is the ridiculous cost of ammo, but if you are talking rounds per deer taken... It's still cheap from that aspect. Certainly no one is using .50 Beowulf as plinking ammo.