On a 79 most any engine problem is fatal. Its a cost value issue. The motors can have a number of fatal things happen after 300k miles. The heads of the valves evenyually wear out. The bushing holding the oil pump drive wears out and the pump stops. Of course lack of maintenance and/or random events cause many earlier failures. You probably shouldn't drive an overheating one more than ten miles, or one without oil more than 3 (vbg). These type of events usually are responsible for early failure of the pistons/rings or crankshaft bearings. Timing chains demand attention and a failure is fatal for that car. They yell their needs and even over the diesel noise they can be heard. Unlike the V8 these engines will rattle for some time usually before they fail.
Pay close attention to your hoses and belts and gauges, they can go forever.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician