I replace the thermostat FIRST, without condition. After that, you can check the radiator by driving the car hard and then turning off the engine and putting your hand on the fins in front of the fan. If the radiator is hot, it is usually OK, the bad ones will have a cold spot somewhere...where the water is not flowing. Lastly the fan clutch. I have only seen them fail on the older cars thus far. Be sure the gauge is accurate and the aux fans work.
The head gasket could cause this, but it is a long shot. If the head gasket is a fault, the pressure inside the cooling system will be high.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS