Low compression can cause white/blue smoke on a diesel, and blowby usually goes hand in had with low compression.

I would see how well the car starts after sitting all night, and once it is started I would loosen the oil filler cap and see just how much blowby there is. If it smokes like a train, the motor is worn out. Don't get me wrong, it will smoke, but it will be obvious if the blowby is bad.

Something to think about is that some '69 220D models had issues with bad motors from the factory, and since the cylinder wall taper is going to be pretty bad with the mileage the car has, doing a re-ring won't last or the rings are going to break if the taper is bad enough.

Pistons are very expensive, so some people re sleeve the cylinders and reuse the old pistons with new rings.

Do not believe the mileage claimed for a minute. I've been burned too many times, look at the car as a whole and forget the miles unless they have full documentation back to day one.