I can only really speak for Mercedes as I own two W124s, but as you said, their classic parts service is very good. Though I feel like it’s gotten a little worse in recent years. Luckily there’s also a huge amount of 3rd party manufacturers. Very specific stuff like wiring harnesses, interior panels and such are best found used on eBay and similar sites. Having the workshop documentation and parts numbers is a must for older cars like this.
I can only really speak for Mercedes as I own two W124s, but as you said, their classic parts service is very good. Though I feel like it’s gotten a little worse in recent years. Luckily there’s also a huge amount of 3rd party manufacturers. Very specific stuff like wiring harnesses, interior panels and such are best found used on eBay and similar sites. Having the workshop documentation and parts numbers is a must for older cars like this.