I've owned a '80 300SD since new and find that its routine, by the book,
maintenance and needed repairs about equal its fuel cost on a per mile
basis. That's about $.045/ mile for fuel plus another $.045/mile for the
maintenance needed to keep the car in very good shape.
Be happy you found a car with extensive maintenance records - you now
know it's been serviced regularly, if that's in fact what they show. A
new transmission is unusual however, ask if a teenager drove the car
before it needed the new box.
This straight six is a good engine that will last for many miles. Its
aluminum cylinder head and headgasket are its weak point. The cooling
system MUST be very well maintained to avoid overheating the engine for
overheating can cause the head gasket to blow and or the aluminum
cylinderhead to crack. A new head is expensive so do the required
cooling system maintenance and don't charge long hills in hot weather
with the A/C running full blast.
Also know that some shops, especially dealers, charge very much, IMHO,
for simple routine work by claiming the flat rate book calls for X hours
of labor when in fact, their mechanic does the job in much less actual
time. Maintenance is called a "C" or "E" service and costs several
hundred $$ but if you look at the bill and figure how long it would take
an experienced mechanic using a lift to DO the work you'll find the
labor charge is unconscionable.
This sounds like a reasonable car and is in demand by diesel fans. If
you buy it have an independent M-B shop undertake a pre-purchase
inspection BEFORE the cash is passed. Then you'll know whether it's a
good value or a list of potential problems. The engine's compression is
the most telling measure of a diesel's health so be sure its included.