I used to have a 300SD/81 for years, mine was abused by the previous owner, did not realize when I bought that car, but ended up paying for all his neglet big time.
Anyway, when MB installed turbo on their 5 cylinder diesels, the engine operation remains non turbo up to a certain rpm, above that point the turbo kicks in and you can feel it in you neck. The initial sluggish start is just the nature of non turbo diesel engine. These 5 cylinder diesels, both 300D's and 300SD's, have a very rough feel in their accelerations, not a fault, but is the way the engine/transmission behaves in normal driving.
These are the last of super reliable diesel engines that MB produced.

Vahe
240D/77 350K