I've heard that the BMW inserts fit. I haven't tried it so I can't give first hand data on that. The MB OE cartridge is still available at the parts counter or through various vendors online (this forum's parent site sells it, not sure if they post to The Netherlands tho...).

As far as the coil resistance goes, the '83 500SL is 15.2Ω, '91 350SD is 14.6Ω, and the spare from my '86 300SDL is 14.9Ω. I'd say your coil is fine.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]