I had a '91 190E and it was always on the warm side. I think the radiator is a bit small for the engine. You might want to track down Jim Forgione's web page. He sells a resistor kit that lowers the turn on temp for the aux fans. Works well for me and our summers hit 118F.
However, if your radiator is old, change it out first and fill with 50/50.
My 320CE has a new radiator but has room for a radiator out of an E420. If you need a new radiator I would think about installing a wider replacement than stock.
Anziani
'93 320CE 208K
'95 E420 126K