They are picky about their maintenance, but if you stay on top of things and use quality parts, they can easily go 750,000 miles or more on the original engine, transmission, and rear end. The biggest problems I run across is people thinking they are helping the situation by performing maintenance, but they're using parts made of Chineseium. Well, that and using the wrong fluids which tears them up quicker than ****. The 2004 to 2006 models are the best, you just have to make sure that it only ever sees 0w-40 or 5w40 diesel oil and Hengst filter in the engine every 10,000 miles, Hengst fuel filter every 20,000 miles, Shell 134 ATF, Fuchs ***an ATF 134, or Valvoline MaxLife ATF in the transmission every 50,000 miles, and a quality synthetic 80w90 in the rear end.
Every 120,000 miles, preemptively change out the glow plugs, the glow plug controller, water pump, thermostat, idler pulleys, tensioner pulley, fan/clutch assembly, and all the turbo piping and coolant hoses. By just following those simple replacement items every 120,000 miles, the vans just really don't break down. The most common problem you will find is random limp mode for a boost leak because they are sensitive enough to de rate the engine if the actual boost is not within three PSI of the commanded boost.
The most common major failure would be an injector taking out a piston, so I advocate that every 200,000 miles an owner should preemptively replace their fuel injectors and seals. It's a tough pill to swallow for an owner, because a new set of injectors is about $2,000 plus the labor, but a remanufactured engine is about $7,000 (if you can even find one). This van in particular had its injectors changed at 200,000 miles, but the previous owner had used cheap eBay remanufactured injectors when he did it. Now, at about 240,000 mi, an injector took out a piston. I don't even have to open the engine up to know this, because it had an uneven cranking speed, and would pop the dipstick out of the tube when you tried to start the engine. Classic symptoms of a hole in the piston.