With respect, sir, "Don't be depressing!"

If the plugs were that bad, replacement may be the cure. But if they had been badly neglected, what about the cap/rotor etc.

So, I stick by what I said. Make sure the ignition system is 100%.

Also, make sure that all electrical connections are clean. I have a policy of pulling connectors, spraying with good quality switch cleaner, plug/unplug to friction-clean the contacts and then refit with maybe a last spray to dissipate deposits. Do this to every connector you can find. It costs little and can be theraputic.

I have a very smooth idle that experts have commented on and this is at 350 K Kms and 18 years old, never garaged - it lives outside in all weathers.

Start with the cheap-to-replace items but don't buy cheap after-market. Ideally, get branded MB parts. Cap, rotor, HT leads may be a priority for you given that the plugs appear to have been neglected.

Bon courage.

RayH