Good deal? I don't know, if he buba'd it and it's missing parts, I don't think so. Sounds like he removed the original mag & mag well to put in a 20 or 30 round mag. You will have to spend extra to keep to bring it back to C&R or it's illegal to use. If he put in the 10 piece conversion kit, then yes, if not then no.
I would look for a Yugo SKS with all matching serial numbers, that would be a better deal.