Plug #5 is one of the easier ones to get at so that's good.
I've had great success with impact drivers. Hot engine, set it on medium torque, and go back and fourth between tighten and loosen continuously until it comes out. expect to spend upward of one hour per plug working it out slowly.

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Current fleet
2006 E320 CDI
1992 300D - 5speed manual swapped

former members
1984 300D "Blues Mobile"

1978 300CD "El Toro"