"Smells pretty bad" - how? Like insulation burning on a transformer that's overheating? That kind of "acrid" smell?? If it is, then run quickly.
I wouldn't pay over a hundred bucks for it, if that. The wire feeder won't be worth much if the power unit isn't working... If all you have to do is clean every connection & the power selection switch/wire feed speed pot, you may be okay. But the corrosion you see on the surface of mechanical parts may or may not be on the transformer windings - depends on the wiring insulation quality. If it is there, and the insulation has deteriorated, run like ____.
And welcome to the GJ from VA!