Rust is really the only unfixable problem. And, if you think about it, the only thing that is different on a diesel is the engine, everything else is the same. Other than the engine you might hit interstellar mileage.

Some issues could be:

Look up " 722.6 K2 bearing " , the trans has a needle beading between the front and rear shafts that fails at high miles. At some point the bearing was upgraded, not sure what year. It might be worth a preemptive repair as when the bearing fails it is shaft and gear time.

The ignition coils ( 2 ) are probably going to fail / should have failed at this point.

The fuel pump is getting to the end of service life. Service life for this part is measured in running hours and not miles. Last year I took apart the "locked up from car sitting 2 years" fuel pump on my 97 C280 / 162,000 miles. The commutator was warn to the point it didn't have many miles left to go. Did the same with a 97 E320 / 200,000 ish , and saw the same amount of wear.

Lower ball joints, boots crack and let water in causing them to seize / break.

Crankshaft sensor failure.