You can borrow one for free at Autozone store. You definitely need the
scanner to read the problem and reset it.
AutoXray has a good product, but is generic... not specific for Mercedes.
This can be had for less than $300... the 5000 model. It comes with lifetime
Another choice would be to use your laptop or Palm with appropriate adapter.
http://www.auterraweb.com ... Again, this is generic so it can read all OBDII
vehicles... from 1996 to today.
Of course there are cheapo OBDII scanner... about $100... but that is
stupid... it only read the DTC number and you have to look it up yourself
and probably cannot do more complicated scan of modern car.
However, it is always free at Autozone. and you can look up manufactue
specific code definition at autoxray.com.