The spark plugs should last 100K miles, so I would have the #6 cylinder spark plugs (there are two per cylinder) closely inspected to see how they look to help ID the problem.

You could just replace the two with the same type if it solves the prob. But as a responsible current owner, most recommend replacing all as a set so the next owner won't have to change out 10 spark plugs in another 15K miles (think about how you would feel if someone did this to car before you just bought it). Showing that you had all 12 replaced would be a good selling point and showed you cared as well.

The plugs are hard to get to on the V6 (at least in the w202 C-class), so expect additional labor cost (over a straight six, or a car that has more room to get to the plugs) if you're paying someone to change them.