A pretty good looking "generic" OBD-II scanner is available
here. I haven't tried it yet but will before long (when my laptop comes in). Should clear the MIL (check engine light) and read engine sensors, but won't do anything for the other systems.
Assenmacher has a handheld device with a module made especially for Mercedes, that will access all the systems (ABS, all-wheel drive, etc.). Should be available from
Baum Tools.
Carsoft says they won't sell to individuals.
Hope this was helpful. It would be interesting to hear from anyone who's tried these or other diagnostic products.