have you ever checked out mercedesshop.com? be sure you type both ss's. it has an excellant forum and searchable archives.
we've had an 83 mb 240D for about seven years. it's a very good car. in my opinion, the steering and front and rear suspension are stronger and more advanced in engineering terms that the volvo 240. the motor, transmission and rear end are very strong.
the down side is maintenence.
parts are available but significantly expensive. the MB is not as simple to work on as a volvo 240. rear drive shafts are more vulnerable than a conventional solid rear end.
if you're doing it purely as a hobby I would say the MB is a better car and thus maybe more fun to work with. if there's any element of practicality involved though I would say stay with volvo.