Originally posted by Adam IMO, "fungus" and "buy" don't belong in the same sentence. Fungus can spread to other lenses and ruin your entire collection if you're not careful.
I can't disagree more. I'd buy a fungus ridden lens if it fulfills the following criteria:
1) I feel confident that it can be cleaned off.
2) It shows no decline in image quality or if the lens was never meant to be that sharp anyway. So, even if it can't be completely cleaned off, I'd know that I can live with the current image quality.
3) It's at a great price.
I don't think a fungus ridden mirror lens is worth $100+. However, I also don't know much about mirror lenses, this might be one of the best ones.
Also, having a fungus infected lens with your other lenses will not increase the risk of infection. It's more of an environmental thing.