I just replaced my dist. cap and rotor a few weeks ago on my 1988 300E W124. Very straightforward. Pull off plug wires from the dist. cap (make sure you note down which plug wire goes to which socket on the cap (well, actually, it's printed on the dist. cap cover (the black cover that you have to remove from the old cap and put on the new cap)), then remove the 3 allen screws holding the cap down, pull off cap...then remove the smaller 3 allen screws holding the rotor down. That's it! The new rotor will only go in one correct way, so you can't go wrong! Then reassemble everything back together and that's it.
The only thing I'm not sure of is whether the negative terminal of the battery needs to be disconnected for this procedure. I disconnected it when I did my dist. cap and rotor. Can any one clarify this?
Hope this helps.