Only you can really answer that question ...
I've purchased a complete sterndrive off Craigslist and I've bought an upper gear case from a salvage yard. In both cases I boiled it down to 1) that's where I could get the parts, 2) I was convinced enough to take the risk on the money and 3) I didn't think it was worth it spending over half the boat's value on a remanufactured/reconditioned drive.
So for me, I'd probably risk the used one on my old boat. Current boat, I might be more inclined to do a rebuilt drive, but ... I don't wanna' know the answer to that question yet