I've replaced an M103 belt without removing the fan, but this was on a car with a two peice shroud that could be removed from the engine bay without removal of the fan. Routing the belt was something of a puzzle, but starting in the tightest spot, between the fan pulley and whatever is at its 7 o'clock position (looking into the engine bay), was the key for me.

Since the tensioning system is new to ejsharp, I'll offer that the tensioner body has an indicator to help you set belt tension. On some cars, the indicator is a series of tick marks, on some cars, it's a triangle. There could be other indicator schemes. In any case, you want the pointer to move from US driver side to passenger side to the end of the marks. I think the pointer stays in place and the tensioner turns, but that's of no consequence. You set tension by tightening the 13mm tensioning nut. When tension is set, torque the 19mm head bolt on the tensioner.

And lay a sheet of cardboard or thick fabric over the radiator before a tool slips from your hand.

Sixto
91 300SE