I owned one about 10 yrs ago,,,an 86 euro (small bumpers) M6 that had some Dinan upgrades pushing it to 300+ hp. It was one of my favorite cars ever ever ever. While somewhat heavy, it was not cumbersome. It would suprise some 5.0 mustangs and z28 camaroes. I saw an indicated 155 mph on a long lonely extended stretch one afternoon and the thing was as stable as a rock. Peppier than you woukld think.
Big thing to check is whether valve springs have been upgraded as this was a common and expensive failure point. Also...if the tranny is the least bit balky, walk away. It had a special Gertag transmission that is nearly impossible to get parts for.
As with some german cars of the era, electronics can have gremlins tho I had none with mine.
that being said, there is a prisitine white 635 for sale in this area for under 10K. Around a 100K miles IIRC. Pretty and cheaper to maintain than an M6. You could probably pick it up for 8-8.5K. That way you could keep the 911
No affiliation...here's the link. Been for sale for a while. Looks extremely well kept:
http://jackson.craigslist.org/cto/1230939360.html