The thermostat should keep the car at about 90c minimum. If you're seeing the temp cool down to 60c while driving, likely there's no thermostat at all, or the one you have is stuck open. No thermostat will also cause the coolant to circulate too quickly, which can cause overheating - the coolant must spend the appropriate amount of time in the radiator to allow the heat it contains to be removed.

One way to check for a missing/stuck open thermostat is to start the car when it's cold. Have the hood up when you start the car to keep the underhood temps from climbing too quickly. Wait a few minutes - maybe 5 at most, and touch the upper radiator hose about mid-way between where it connects to the radiator and where it attaches to the thermostat housing. BE CAREFUL of moving parts, as this must be done while the motor is running. The hose should be at or near ambient temperature. The hose should remain relatively cool to the touch until the thermostat opens, at which point the temperature of the hose will change rather quickly. If it gets warmer very gradually, then likely the 'stat is stuck, or is missing entirely.

Since your A/C is out and the aux fan is not running, you will likely see increased temperatures in traffic in warmer weather, but I'd think that the engine fan, if the viscous clutch is working properly, should be able to keep you out of the 120c range - other's may have different ideas here, I'm still learning about my Benz. I've read several posts that talk about the "rag test". I assume this to mean that if the clutch is operating properly, when it's fully engaged (I'd guess at about 100c on the temp gauge), a rag, when placed across the shroud of the electric fans, should be held in place by the airflow. Others suggest that a fully engaged clutch will cause the fan to stop spinning almost immediately after turning the engine off.

I'd check/replace the thermostat. The work is fairly easy and the parts aren't that expensive. Modern cars MUST have a properly functioning thermostat to run properly. Just be sure to follow the instructions carefully on replacing the 'stat on your car. I missed one little line in the instructions when changing mine and had a devil of a time getting the air out of the system.