If the cat is bad, which it might be just from age - say a 5 yr old stove, it would need replacing anyway. The cat is a consumable item - replacement isn't a big deal. The woodstock cat's are/were around $125, they are warranted for I think 6 years, the first couple 100% free replacement.
With regard to used cat stoves, the most potential for problems are the Vermont Castings stoves due to their fragile refractory guts. You can get a good used VC stove, but just be aware you might have to plow some $$$'s into it if the refractory gives out. I think another used/easy on the eyes wood stove would be the Dutchwest cat stoves, but oddly enough, I'd stick with the Vermont Castings built ones vs the much older Consolidated Dutchwest versions. The other makes of cat stoves, like Blaze King, Buck and Woodstock, I think you could buy without to much worries - other than if the previous owner overfired them and warped something. None of those stoves have fragile refractories to damage. The Dutchwest has a refractory around their cat, but I don't think it is possible to physically damage it so it's an OK application of that material. For the ultimate in reliable, basically new but used wood stoves, Woodstock has refurbish stoves for sale from time to time. These are stoves that customers returned under their six month money back warranty. Todd bought one of these and from my understanding of his comments, what he got, while a refurb, was a new stove.
Do some searching on cat stoves - especially the Woodstock ones. I think you will like them.
Good luck,
Bill