stay away from cheap steel cases unless you like swapping extractors.
We've run almost nothing but steel our AR's and have had no such problems at all through many, many thousands of rounds. The bigger issue with surplus rounds in a 7.62x39 AR is hard primers with a standard AR firing pin which is slightly too short to reliably ignite them. A heavier hammer spring has worked good for us, though I know some guys actually modify their bolts by taking a few thousandths off the end too allow a little deeper penetration of the firing pin for those harder primers.
The biggest issue you'll have is finding reliable magazines, because they're just a lot more scarce on the 7.62x39 AR platform. There's a great sticky posted here
on this caliber in the AR that will probably answer most of your questions and help give you some other ideas as well.