Lots normally. A diesel engine itself is pretty durable - they typically last longer than gasoline engines (if maintained regularly anyway). On a car with 200K mi on it I'd probably be more worried about other components wearing out before the engine itself. Things like transmissions typically wear out before engines, especially in diesels simply because diesel engines last so long. A new transmission is very expensive in any car.
You might save some money on fuel due to better fuel economy of the diesel (or you might not save much - diesel fuel itself is expensive) but any savings on fuel will be quickly eaten up and then some in repair costs if a major component like the transmission goes bad.
Probably about 100,000 more. They run a ling time. The German 80s Mercedes in particular are TANKS, and anything Diesel is made to go the distance.
You must, of course, take good care of itm change all parts at regular intervals, and make sure your coolant does not corrode engine block passages. If you do this, and keep the engine clean. it should not fail for a long time. Corrosive stuff can kill it, though, its a tank but its still metal and steel.