Never really thought about Inskip. I'll try them for a part order soon and see how they react. I did find out the Viti Bros. MB in either Tiverton or Middletown is pretty trustworthy. Don't know if they'd work on the old cars. Since I wrote the question, I've obtained 2 300D's, both 83's, one sedan and one coupe. So far I've been able to do all the work myself. One or the other of them will have its A/C fixed this summer. That I do have to farm out as I'm not an A/C knowledgable person. The W123 cars are not too bad to work with and parts haven't been a problem. I try to buy what I need here to help support the site.
As for German Motors, I brought my '58 220S to them once to see if they could balance the carbs. Basically the guy told me his father (semi-retired) might take it on just out of interest. They didn't seem too helpful.