I have a '91 300E. Since plugs are so easy to change and so cheap to buy, I just arbitrarily change them around 20K. This varies depending on whether or not I'm going on a long trip or about to have an emissions test.
The most difficult thing about changing plugs on the 300E is getting the plug wires off w/o damaging them. I discovered that if you spray the inside of the plug wire boot with HD silicon spray, the wires will pop right off the next time you change plugs.
Bosch copper plugs are pre-gapped and have an anti-seize compound already applied.
Mercedes recommends that plugs be checked at 15K and changed at 30K. These plugs are so cheap and so easy to replace that it's silly to not just pop in a new set.