Make sure your thermostat is operating properly first. It only take 2 minutes to remove. My 2.6 had the same symptoms and it was the thermostat. It was puzzling because it would open just a crack and the circulation at highway speeds would get the temp around 80 and would rapidly go up past 105 at a stop light or in traffic. I thought it could'nt be the thermostat at first because it never would boil over. The thermostat is about $25. If you are going to do the fan clutch, you are going to need to purchase at least one special tool or improvise to get the fan clutch off. There is a special tool that holds the fan pulley so you can remove the center bolt. Also a special short hex socket to get the fan allen bolt out, as you know it is very tight working on the front of the engine.
It is far easier to start with the thermostat. The fan clutch is not a fun job.
1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T