XE-1 is a nice body. No reason to let it just succomb. I wouldn't bother trying to clean the mirror on your own (although you might not have much to lose here), but in general I'd expect any of the typical mold killing regimes (vinegar/bleach [might be iffy]) to work out here. However since the interior might have fungus/mold, you might just consider getting it CLAed for a reasonably price. Since you were gifted the camera, I'd just count it as the cost of ownership.

Seeing as the camera was once someone else's pride and joy, I'd impart the same level of respect to it: don't write it off.