My only contribution to the already excellent advice above is to realize you're buying old iron- and all the problems that come with it. The SB 9 lathe didn't have hardened ways, so most likely you're going to get wear (perhaps significant) in the ways. Parts are pretty available on ebay, and the rebuild *IS* a fun, rewarding experience. But like most things that are 70+ years old, you're going to have some age/wear problems with it that you need to be OK with.
I have no regrets in purchasing/restoring my SB 9a, but certainly I learned a lot of lessons in doing so. I'll have a better eye on things for my next purchase.