I didn't know the M103 made it to 93. If you're not familiar with this engine, be ready for the most complex belt tensioning system you've ever seen. I can only wonder what the newer engines are like.
In any case, to remove the belt, loosen but do not remove the big headed bolt (19mm?) in the center of the tensioner pulley just to the right of the power steering pulley. Then relieve the belt tension by loosening the tensioner. The tensioner is that hollow 13mm hex head thing sticking up behind the tensioner pulley.
On the tensioner body is a set of marks used to set tension. On mine, there's a pointer that moves with the tensioner, and a set of marks that look like a right triangle and a post (hard to describe in words). As you tighten the tensioner, the pointer will move along the triangle. Proper tension is set when the the pointer is between the triangle and the post. If the belt still feels loose, then the tensioner is bad. Which makes me wonder why MB employed this system.
There's a little shock absorber just beside the oil filler cap. It is part of the belt tensioning system. I don't know the consequence of a bad shock absorber, but it won't be a loose belt.
Take note of how the belt is routed before you remove it. It will go on the wrong way... maybe several wrong ways. There is also a sequence for threading the belt on the pulleys. I don't know how critical the sequence is. I got my belt on without following the sequence, and it's held for almost a year.
91 300SE (M103)