If you do end up doing a 7.62x39 just a few pointers, if you want to consider in the build.
1. I would stick with carbine gas, if you desire to run steel case, to help with the dirtier nature.
2. Run a heavier buffer H2 (H3 if it will run will be better) this will increase dwell time allowing for pressure to subside a few more milliseconds to ease wear on locking lugs.
3. C-Products magazines work great, I really only use the 5 rounders for hunting and 20 rounders for range. I've heard to stay away from the 30 rounders but that the 28 rounders will work.
4. A big part of getting the 7.62x39 running right is having a good wide feed ramp in the barrel extension, early on in the life (we are talking 15 years ago) of 7.62x39 barrel manufacturers were using standard 556 feed ramps and were having major issues. Barrel manufacturers today probably have all realized this and don't do that anymore, but it's always good to double check with the barrel maker to make sure they accounted for a 7.62x39 feedramp in the extension.
It is one of my favorite AR chamberings, it really is one of the perfect cartridges for starting young kids out with hunting, mild mannered and great performance with handloads.