Thanks to M.B.DOC.
The spark plug tip from you (F8 DC4) resulted in a fully operational SLK200 (non-compressor engine).
It is hard to imaging that the spark plugs could cause so many problems, like rough idle (fluctuations), reduced acceleration/engine power (hesitation) and misfire under load. The fix for these symptoms could have been really expensive.
As learned from this forum the F8 DC4 plugs were gapped to 1.0 mm. The standard gap that is stated in the Bosch manual is 0.8 mm.
The “new” gapping of the plugs resulted in a better (more stable) idle, better cold start, better acceleration (no hesitation). The misfire under load is also history.
The power of the engine at low rpm’s (in the beginning of accel.) is also, without doubt, increased. The engine is smooth and responsible and the car is again a pleasure to drive.
The learning for me is the following: “Do the cheap and easy things first - then the expensive ones”. I have seen this phrase several times in the forum and in my case it was valid.
…and again thank you for this great forum.