I too am in MI (The Great
Cone State). I found you can get a lot of Boxster in the upper Midwest for $15K these days. I got my "new" 2001 (non-S) with 42K for $12,500; outstanding condition and perfect service history. I think I got a good deal, but I drove several in that price range that were comparable.
If I had to do it all over again, I would focus my search solely in the North. I spent a lot of time in the South and West looking at cars. Most were driven year-round and often parked in intense heat. Typically on the early models it really shows in the interior. I didn't find that additional year-round wear and tear was factored into the price to the extent that I thought it justified as I ventured South and West. Just one man's experience.
One of the (only) advantages to living where we do is that Boxsters are almost strictly summer cars/secondary and mollycoddled as such.
Truly sorry to hear about your car but in this economy at $15K you may come out ahead or at minimum on par (not counting hassle factor, of course).