Now would be an extremely poor time to buy a pre-ban AK IMO, unless you happen to find one for a great deal (ie down near post-ban AK prices...).
That $850 isn't a bad price for a pre-ban with a 75 rnder, but like I said now is not the time to be buying one. You should be able to get a AK equipped similarly, likely made better (I'm not a chinese AK fan anymore...), for cheaper in a month or so (gotta allow some time for new production evil feature AKs to get into circulation after the AWB sunset...)
If you want an AK right now, it would make more sense to me to get a current postban. Then in a couple weeks you can pick and choose which "evil features" you want to go to the trouble/small expense of adding back onto it... You can rent a barrel threading kit for $10-$15 for example (if interested there is guy on the gunsnet.net AK forums renting these), and folding stocks are already cheap and easy to come by, one could be added easily (A underfolder would be a little trickier, but doable...)