The charcoal filter is located under the dash above the passengers side footwell. I haven't seen the FSS sheet for a 203 chassis yet. On everything but the 140 chassis, it's replaced every 45,000 miles, the 140 charcoal filter is every 75,000, but the parts and labor are much higher on the 140. I believe just the C320 has the charcoal filters, not the C240 or Sportcoupe. 209 chassis CLK I believe all have the charcoal system.
I can see if you want to do additional oil changes, I assume you are still gettting the FSS service (A and B) done at the dealer for free.