I would recommend getting an AK and adding a folding buttstock and forend rail. If you want to shoot the 7.62x39, do it out of the proper platform. The ARs loaded in 7.62x39 have reliability issues related directly to the design of said round. The primary issue is with the magazine. The 7.62x39 (and the 5.45 also) are designed with a tapered cartridge. This design requires a constant curvature of the magazine. This constant curvature is reduced in the upper portion of the magazine due to the mag well being straight. You can get around this for 5.56 by using internal curvature, but the 5.56 is a much less dramatic curve than the Soviet rounds. The 7.62x39 requires a much more dramatic curvature, and lack of it can cause binding in the magazine and reduced feed rate, which can cause loading malfunctions, or bolt over-run (bolt closing on an empty chamber because the magazine can't feed fast enough to keep up). The other problem is with bolt-thrust issues, which are created by the power of the 7.62x39 round. The AK round is much heavier and creates high pressures while also having high inertial recoil onto the bolt. As a result, this causes a larger issue of the bolt unlocking early and throwing off the timing of the BCG and gas system. As a result, you'll have increased instances of failure to extract/eject and get a double-feed, or you'll have failures to feed. The backward bolt-thrust of a 5.56 is around 6,833ft-lbs, while the 7.62x39 is 7,918ft-lbs. There isn't a heavy-enough buffer or recoil spring to eliminate this issue, unfortunately.
The final problem is with the design of the rear case head and the rim of the AK round, which is just not conducive with the design of the AR bolt.
So, just eliminate the problem and get the rifle that the round was designed to be shot out of. If you want a .223/5.56, get the appropriate weapon system and get an AR or similar combat carbine like a SCAR, ACR, or Sig556. Don't get AKs loaded in 5.56. If you want a light AK, get the AKM variant with the 5.45x39 chamber.
Trying to mix rounds is just problematic. Don't compromise. It's not a compromise to get an AK with some AR furniture. That's just changing the ergos to fit you. Getting an AR in the 7.62x39 is a compromise, and one that usually does not pay off well. If your issue is with cost, then the AK is going to be a better option. The maintenance required on an AR is pretty minimal (if taken care of and lubed properly with good lube), but the AKs tend to have a lower cost of ownership due to less accessories and stuff being available.
I guess I shall vote AR with AK components, although it won't be as purty a gun. 
