I do not know the size of the wrench you will need but my guess is about 8 mm.
I have yet to remove a starter from a Mercedes but for those starters that I have removed, I always needed an extension for the rachet socket for the back bolt.
If this is the case with the Mercedes then you cannot use an L shaped Allen head wrench, you will need a hex socket that will fit on the end of an extension bar. Or if you have an old Allen head wrench that is the right size you can cut the end of it off with a cutting wheel and then insert the cut off into one of your existing millimeter sockets. Otherwise you can pick a hex head socket up at Sears or other tool store.
[Edited by ejsharp on 05-27-2001 at 12:22 PM]
1993 190E 2.3
2000 Toyota 4x4 Tundra