Just bought a 2001 S55 AMG with 67000 miles. Very clean, been maintained at BENZ dealer all along. Only issue I see so far, which I am not sure is significant, is that the temperature gauge starts normally low in the morning and then levels off around above 80 but sometimes fluctuates between 80-100 but never reaches 100. No leaks, no noises, no overheating, not sure what the ideal temperature is. Not sure if thermostat been changed.
Temperature gauage strange on 2001 Mercedes-Benz S55 AMG
I would do a cooling system inspection. Make sure that your cooling fans work properly and that the system holds pressure. Then I would flush the system and replace the thermostat, only use Mercedes Factory Coolant and a factory thermostat kit. This is important.