If it's nothing more than the drive shaft to the blower broke it's not that hard to repair. All you do is pull the cover plate off the rear housing that's inline with the blower, remove the clip that holds the shaft in, and remove the broken pieces with a good magnet. That said, if the drive shaft is all that's broke then you might be in luck and it'll be a cheap and easy fix. The only problem is that usually when a dirve shaft twists off/breaks it does so for a reason.
The last one I saw that did it because a liner split in two and threw a small chunk into the blower locking it up. The engine was running wide open when it happened so when the blower locked down the engine still had enough momentum to going to twist the shaft off. In that case a complete engine rebuild was needed.
I also have seen them where a blower bearing locks up and the same thing happens with the blower drive shaft being twisted off. In that case the engine itself is OK, but the blower would need to be rebuilt. While not really cheap to have done, it is still alot cheaper than a complete engine rebuild.
My advice would be to get a Detroit mechanic or at least someone that knows something about them to take a look before you make the purchase. I say this because there are quite a few things to look at in regard to the problem you describe, and as the examples I gave show, they can run from REALLY EXPENSIVE, full rebuild, to just an expensive blower rebuild, to the cost of just a shaft, a gasket, and a few minutes to replace them. Good luck and I hope this helps.