The original plug wire ends were a flawed design, replaced with a new design. You should be able to take the part number to the dealer and see if you have the new or the old. If you have the old it is possible to replace only the ends but the wires will then be too short to allow for reinstallation of the black spark plug hole covers. I left mine off and the car was fine for years but obviously the right way to do it is to replace the wires and the ends. I've always thought of caps and rotors as about 60k but am not aware of any official MB position. Maybe your service book shows it?