This is exactly what I did, I got an old MT-2500 scan tool from fleabay for around $300 and then ponied up $550 for the MB specific cartridge kit from a local Snap-on dealer. The kit I bought contained everything needed (software, cables and personality keys) for Mercedes from around 1990 through 2003. I was somewhat hesitant to spend close to $1,000 on a scan tool but my thought was "I'll be driving a MB car from this time period for at least another decade - I should be able to make up the outlay over that time period"

The Snap-on tool is pretty good for an advanced DIY'er - It will read/reset codes from most of the subsysterms (engine, transmission, climate control, SRS, ect, ect) read adaptations (engine and transmission) has a variety of actuator tests (for example I can manually activate the EGR valve or smog pump, selectively cut-off fuel flow on a particular injector) and it has a wide variety of real time data that I'd guess is read off the CAN.

The MT-2500 is no substitute for the SDS or HHT but it's capabilities are way beyond your garden variety OBDII scan tool. AFAIK at the the $200 dollar level you are going to get some variation on the standard OBDII reader.