You've come to the right place.
Diesels are essentially constant speed engines - yes, they accelerate,
but that's not their forte, so diesels of this age seem like slugs
compared to gas engines (the new, 2007 MY, CDI diesels are much better).
A diesel is a compression engine and depends on its compression to
ignite the fuel (as opposed to a gas engine that uses a spark). So, a
diesel engine's health is judged by its compression, which can be tested.
I'd urge you to call a few independent M-B shops and find their cost of
a "prepurchase inspection" and if it includes a compression test. That's
what you want, then you'll know whether this engine has a lot of miles
left in it - or not, in which case you should pass on the car.
If you buy an older four or five cylinder M-B diesel you should know its
Engine oil change and filter every 5K miles, not complicated, a DIY job.
Engine valve adjustment every 15K miles, preferably in the fall, a DIY
job with instructions.
Engine air and fuel filters (2) every 30K miles, a DIY job.
Transmission fluid & filter at 30K miles can be DIY if you can work
cleanly, or shop job.
Change coolant every two years, DIY.
Replace brake fluid every two years.
Many owners DIY because the cars are simple and understandable.
Parts are available on-line and through M-B dealers at higher cost.
These are very good cars, simple, solid and reliable, provided you don't
skimp on parts' quality.
Ask the seller if she has the car's maintenance records which will tell
you if the maintenance schedule was observed or not and which shops did
the work. The more information you get the greater your confidence will
be in the car.
Come back anytime with questions.