Visco clutch locks up before the aux fan runs in high speed. Ditto on my 280 SE -- visco clutch locks up at about 95, aux fan at 100C.
These cars are engineered to run anywhwere, not just Germany.
The usual problems with cooling start at about 200,000 miles on average, at which point the radiator is full of crud and needs to be replaced.
Don't short the engineering expertise of MB -- their cars are way ahead of everyone else. Not so true now as it used to be, but still true. Overheating is a maintenance issue, they DON'T over heat when in good condition. 120C is not overheated, and the timing is not retarded when hot, it is advanced to speed the engine up! Knock will retard the engine (use premium, the best you can find, in hot weather for this reason). If the temp gauge sits at the mark above 80C all the time in traffic, your radiator is crudded up or the visco clutch isn't working.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!