100C? What's the problem? As long as it never looses coolant and doesn't peg the needle in the red, I wouldn't lose any more sleep. You'll achieve better economy with this engine running at 90C (193°F) than at 80C (176°F) and, it's better for the engine to run a little hotter too.
The gauges in these cars are notorious for being poorly calibrated. SWMBOs 300D and my 300TD both have new, identical 90C thermostats (tested to open at 90C in a pot of water on the stove), freshly flushed cooling systems and all new hoses yet her car always indicates 100C and mine always indicates 80-90C.
If you're really fretting, I'd pay another $99 and buy a non-contact thermometer. Then, go run the snot out of that car, stop in a safe place and take the temperature of the top radiator hose or at the head where the temp sensor screws in. That will be the REAL temp.
Raytek non-contact thermometer:
http://www.contractorstools.com/raytek_mt2.html