I too have been looking for an inexpensive, yet useful OBDII scan tool. I think I may have found it....
http://www.obd-2.com/

The software is free to download and take a look. It runs on any old laptop you may have lying around (even old 386 machines).
For $122 he supplies a cable to hook to European models with ISO compliant plugs (includes Mercedes).

I downloaded the software and took a look, it seems pretty good, and I like the logging features to track down "mystery" codes. I have an old Toshiba laptop that just might fit the bill.

If I order it (and I think I will soon), I will post a mini-review of it once I figure out all the features.

-Earl