Over 100C is normal in hot climates. The temp on MB thermostats is the temp at which they start to open. A lot of the time, as thermostat gets old, it starts to open at a lower temp.
If your temp is holding steady at 80C, you need to replace the thermostat. I replaced mine when I changed the coolant, and now it is back in the upper 80s to 100 range. Before I replaced it, the temp was running 82C on the highway.
Think about it, if MB thought that running 100C was not normal, then why did they design it so that a properly functioning thermostat should still be restricting coolant flow into the radiator at 100C? The fan clutch doesn't even start to pull mass air through the radiator until 95C.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record