Mechanically other than crud in the starter it could be the Over Running Clutch is shot or the Ring Gear Teeth are all burred up and keeping the Starter Gear from Engaging. But, you will see the Ring Gear when the Starter is removed.
The burrs can be filed or ground off if there is room to get at them.
I don't know if we have specific instructions in the DIY section for your Year and Model.
DIY Repair Links
DIY Links by Parts Category - PeachParts Mercedes ShopForum
PeachPartsWiki: Do It Yourself Articles - Mercedes Vehicles
Disconnect your Negative Battery Cable for sure on this job.
If it has Allen Head Bolts like the earlier Models you need to be sure the crud is cleaned out of the Head and that the Allen Wrench End of the Socket is does not have too much or a rounded end on it. If you want to get the Full Bit of the Allen Wrench end you need to grind it flat.
On some of the Models there is limited room to pull the Starter off and outonce it is loose. Some have had to drop the Tie Rod or other Steering Rods.