To date, just exactly one year, I have replaced a bad thermostat, replaced the vac cutoff, replaced the pressure valve seals in the IP, and fixed the sunroof. I have also disconnected the vac lines for the air recirc and EGR since the actuators are shot. That's it (the tire repair isn't MB related). Oh, and I changed the oil, too. I've put about 8000 miles on it. Spread over three running cars, I don't put much milage on any of them!
All horror stories aside, if you maintain these cars properly, they run forever. You can expect to need a head gasket in the 160,000 mile range (or interval, actually), and watch that rad -- mine was already replaced when the PO bought it (I got a $2000 discount because it didn't run right, the pressure valve seals fixed that).
There are still a few fiddly things wrong -- I need to finish the sunroof (I have to paint the cover and around the seal), one headrest is broken/stuck, the cargo nets were ripped off the back of the seats, and the headlights are strange, but that's it. No indications of further trouble.
As far as blowby goes, all cars have some, and diesels tend to have a bit more. The oil collects in the turbo intake, and while it may appear that there is a lot there, mine was wet (using a quart or less in 6000 miles), there didn't appear to be any on the compressor. The Volvo uses about six times as much, and the compressor is definitely oily. New cart, though, so I know it isn't the turbo.
You will get oil in the intake because it gets sucked out of the engine when the turbo comes up. You will DEFINITELY notice the turbo on this car -- feels like a JATO bottle! If it doesn't stand up and roar at 2100 rpm, the turbo is shot!
B2 piston is a very typical problem on these cars, but look at it this way -- on a GM you would be replacing the entire tranny, not a pison seal! My Chevy work van is on tranny #3 at 295,000 miles. No engine work though.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!