The A/C system is tied into the car's electrical system in several ways. I'd not waste my time with any shop unless I KNEW they were familiar with MB cars. I'd find an independent shop known for MB experience for this.

If the fan in the passenger compartment AND the engine fan don't start (and you've made it clear that the A/C clutch does not engage) I'd ask you if the red light on the A/C button comes on. If not, this is a controls issue and I don't see how the average auto A/C shop is going to be any help. Try following the schematics in the manual yourself and you'll see what I mean.

You need someone experienced on MB cars.