Congrats on the find -- low miles!! I'm envious!!
The dash temp gauge is not a finely calibrated instrument. Use it only as an approximation for the engine coolant temperature. Don't rely on it for absolute temperatures. It is more useful to show trends, such as when the engine warms up, what the steady state temperature is, and when it's beginning to overheat.
I agree with haasman. The only good way to check out the thermostat is to remove it from the car and test it on the kitchen stove it in a pot of water.
Buy the Haynes manual until you can find a better one. The money you spend on these things will pay for themselves many times over.
95 E320 Cabriolet, 131K