As a general rule most MB diesels have reliable power train and very unreliable luxury features, as these 123 and 126 cars get older it pays to stay away from the fancy models.
At one time I owned a 300SD/81 and 24D/77 manual, the SD is long gone due to continuous expensive maintenance, the 240D is still running just fine.

If you live in a relatively flat area and do not need year around AC then 240D is a nice car to own, I would definitely stay away from the automatic 240D and also the older models with York compressors.

I get 31 mpg on highway from my 240D, it has a new motor with 60K on it. On highway I generally stay around 65mph.

Vahe