I can tell you that the C280 should not be below 80C. I have seen this car sit just over the 80 mark in 20F weather at 70+ MPH on the highway.
Someone once posted a way to get a reading of every sensor in the car through the climate control. To get a more accurate temp of the coolant, hold down the rest button for about 10 seconds with the car and climate control on. Then it will start flashing two numbers in the temp part of the display. The first number will be the number of the sensor, the second will be the actual reading. Each sensor is numbered in sequence. Use the blower + and - to cycle through to number 6 or 7? That should be the coolant temp. In the 97 C280, it is Fahrenheit, don't remember the exact number of what it should be, but it should make sense.