Both camera makers understood that a Contax or a Leica would neeed to be serviced. Today, both are reasonably simple to service.
A Contax isn't that complicated, although it does take time to disassemble and reassemble.
I don't believe any of the parts are interchangeable between a Contax I or II and a postwar IIa/IIIa. The only thing that is the same is the specifications for the lens mount to ensure lens compatibility between models. Except for the prewar Biogon, which shouldn't be used on a postwar Contax.
Some prefer the viewfinder of the Contax II. My own preference is for a postwar IIa. The IIIa is a good camera, but I find that the meter addds noticeable weight to the camera.
The early Nikon S rangefinders are well made, but I think that they are heavier than they neeed to be.