There are hundreds of 200/700/940's with more than 300k miles and never had the engine rebuilt, much less than even a head gasket, engines were never opened.
The last 3 240's I have had, have near or over 300k miles, never opened, never needed a valve job, no piston slap, no loss of compression, no leaks, etc, including my current dd which is a '91 245 with 310k miles. All of these cars were purchased from the original owners and between 150k and 200k miles when I got them. Each one got a complete Stage 0 maintenance of every system or wear part on the car, then drove it another 100k mi plus.
If you simply change the oil every 5000 miles and stick to the 30k tune-up and fluid change schedule they will last nearly forever. I have used synthetic oil but a good diesel or synthetic blend oil is just as good if you change it regurlarly. I am currently using Cheveron Delo, and Valvoline Durablend was my choice for years until they stopped making it.
One other thing which may seem to be a no-brainer to some, and ignored by others, is keeping a close eye on the intake, hoses, flame trap, throttle body, etc and maintain good engine vacuum. When these are let go, and blow-by is present in an older car, the engine suffers to maintain power output, and struggles to keep the car up to it's usual standards, runs hotter, and engine wear can occur, so maintenance there is the key.