On my W202 (a 1995) I have a silver circular cannister with a black nipple on top. If you unscrew this, remove the metal lid and look inside, there is a white plastic part that indicates the min and max levels. On the metal lid, there is a rubber gasket. That part is about $3-$5 to replace, and could literally be done by a drunk monkey. I think, however, that there is a seal between the reservoir and the pump that is not as easy to replace/renew.
On newer W202s, instead of a silver circular cannister, there may be a squarish black reservoir with a screw-off knob-type top. I believe that this may have a dipstick built into it, but I am not sure.
Also, on a side note, if you are adding fluid, don't use ATF, add PS fluid. Some people recommend using genuind MB fluid, I use Febi-Bilstein, and that seems to work just fine.
98 e430, 55K