No I don't no anything about the C320, other than they seem like a nice little car.
100k seems to be the point where value drops slightly, although it doesn't mean the car is old. Just the general buying public views 100k as old. People like me on the other hand can do pretty well snapping up 100k cars.
I wouldn't sell it because you think its value is going to drop, it already has. By keeping it a few more years not much more will be lost in resale. Maybe $1k or $2k. I have seen mint 60k-70k 1997 E420's in the $15k range. I have also seen 1998 E430's with 130k on them in the $13k-$14k range. I was just doing pricing research on V8 powerd W210's because I am thinking about getting one. Try Autotrader.com good site to use lots of cars on it.