Replacing the fan pulley bracket isn't particularly difficult. There's a tube that runs in front of the bracket and to the thermostat housing. You probably should have a new o-ring ready if you have to remove the tube to access the bolts of the bracket.

The tensioner has a gauge to help you set belt tension. Relative to the tensioner body, a plastic pointer goes from right to left (as you face the engine from the front of the car) as tension is applied. Proper tension is set when the pointer is at the thickest or tallest point of the triangle on the tensioner body. The indicator style varies from year to year so yours might not be a triangle as I described.

If the belt feels loose when the tensioner indicates a proper setting, the tensioner should be replaced (makes me wonder about the effectiveness of the indicator).

There are several threads with comprehensive instructions on this procedure. Do a serach for terms like 300E, M103 and serpentine with belt. You can narrow the seach to those with input from a user who goes by jeffsr.

Sixto
91 300SE
81 300SD