Not the cheapest at around £150-£170, but it does ODBII plus Merc-specific components i.e. ABS/ESP, SRS (Airbags), etc.
It reads codes and resets them no problem.
I do recommend this unit, however the Merc-specific bit only goes up to W203 so if you have a W204 (I am assuming you have a C-Class?) it will only do the standard ODBII codes but not the Merc-specific stuff.
(the unit's firmware can be flashed for 'free' updates, but as no updates are available this from the manufacturer is not really helpful)
It's quite good, there is room for batteries but it does not need them - when connected to the car it is powered off the ODBII plug, when connected to the PC it is powered off the USB port. The batteries are just in case you want to view stored data after disconnecting the unit from the car.