In my experience if fully serviced on schedule they just keep going, diesels especially. Mine has 170k miles and is as tight as when it had 50k on it. I expect to run it to over 500k. I know a taxi driver in Belgium with a 300D who runs it 24/7 on shifts and he has over 1,000,000km with original engine and gearbox. Key issue is service history, i would rather buy one with 150kmiles on the clock and a full history , few owners etc than one with 50k and no service record.

Keith