In the US, only the 500SELs and 560SELs (and the coupes?) have leveling suspension. The rest of the 126s have conventional springs. Some US-spec cars were special ordered and the gray market was booming in the early and mid 80s so it's possible to find anything.

You're new to MB Diesels I see What you want is a well cared for never opened MB engine. The bottom ends are good for 4-500K miles. And many heads will stick it out for as long. The time and care it takes to rebuild an MB Diesel (or any MB engine for that matter) makes it a cost prohibitive proposition for in the general sense. It can be done for less, but will the results be worth the effort?

Transmissions are another matter. 150-200K miles is about as far they'll go even with proper care but there are exceptions.

The problem with your budget and requirements is that a proper engine and transmission rebuild will cost more than $6000 so if someone sticks their wallet out that far, they won't be eager to take $6000 for the car.

Sixto
95 S420
87 300SDL