The delicate center carbon contact is easily damaged when installing the cap due to the high ridge that it has to clear on the rotor. Cap must be forward enough to clear and then installed straight on. Room is very tight especially on the W201.

As far as the availability of none resistor plugs, I buy online NGK BP5EFS plugs for my M103 from one of the UK sellers. Price is excellent and shipping to Canada is very reasonable (unlike from the US) and I get them in about a week or even less.
I've been using these NGK plugs for about 10 years and I'm happy with them.

As far as replacing the alternator regulator and wires every year, unless you are doing 100k miles a year, it appears like an abuse of a lifetime warranty (no offense intended).