Besides not working on anything but engine management, an OBDII scanner is just a fraction of what can be done with the proper tools.
The real ability to diagnose the modern system comes from being able to manipulate the system while watching the proper data. There is no manipulation with OBDII. Even the data is mostly irrelavent. MBs have about 4-5 things involved with thenthgrottle action. A OBDII will be lucky to give more than a single PID for throttle.
A proper scanner would allow one to view the variation in power driven crankshaft acceleration rates (misfire). The number has a threshold, which when passed will trigger an OBDII P03xx code. With OBDII thats all you get is the code if it registers. With a proper scanner one can reduce the fuel by 7% increments and see if the misfire factor changes on a cylinder (or more or one can add fuel - either way to over 29% difference). The same can be done with timing control.
Evap, EGR, and Secondary air codes are about impossible without the ability to engage the systems and monitor the critical factors, none of which can be done with OBDII.
I would say that having an OBDII tool is about 10% of what a real tool will do on engines and nothing else. if one were to want a tool that did about 70% on all the major systems for a serious DIYer (one not just in it for the money) one should buy a Snap-On MT2500 on ebay (a complete kit cost me $4k in 1995) for less than 500 and then buy new or on ebay the MB module. New could be almost another 500.
The best deal going on the real thing is the BASIC from MB. it goes for about $9500 plus a few thou a year for updates.
Bosch Master, ASE Master, L1
33 years MB technician