The answer to your question depends on what year and model of Mercedes you have. The older E series cars (190, 300 and 320) were fitted with a belt tensioner that required you to loosen a lock nut on the tensioner and then release belt tension by backing off an adjustment bolt. The newer cars use a simpler arrangement where the tensioner is just a spring loaded pulley mounted eccentrically. To loose the tensioner, just use a ratchet and a 15mm socket to "wind up" the tensioner while you remove the belt at the same time.

None of the above cars require that you remove any of the accessories in order to change out the belt.