There was a brief period where diesels benefitted from the new technologies, until the other newer technologies were imposed on them by government. I am speaking primarily of the 1994-2006 5.9 Cummins, the 1995-2003 7.3 Powerstrokes, the 2001-2007 Duramaxes, and the 5cyl Sprinters.
During these years, there was no DEF fluid gumming stuff up, and all of these designs (if maintained properly) were reliable and long-living.
Redesigned to burn the new fuels imposed by government was what caused the famous problems with the 6.0 and 6.4 Powerstrokes, and even the 3.0 Mercedes V6s that came after the 2.7 5cyl.
As has been said, when buying a used diesel van or truck, previous maintenance history is key. If it can't be proven,...it never existed. There is a unique paradox of maintenance that usually occurred with commercially driven vehicles, especially the Sprinters;...if it was owned by an individual, such as a freight contracter owner/operator like the Fed/Ex independent "partners",...the vehicle may not have been abused or driven as hard as an mere employee of a large corporation who has no ownership stake in the vehicle, but because the owner drove it easier and was always counting pennies, they tended to skimp on maintenance and not always keep up timely on what should be regulalry scheduled check-ups and routine oil changes and such. Whereas the larger corporation might have a well staffed mechanics garage for attending to the vehicles and even get them up on a lift more often just for occasional inspection and preventative maintence schedules,...the drivers who were just poorly paid and maybe poorly trained young bucks who didn't a crap, drove it like they stole it and couldn't care less if something broke.
All that said,...as long as the vehicle has been driven by sane people who took care of them,...diesel motors tend to last AT LEAST twice as long as gas motors. Thus, if you think 100,000 miles on a gas powered vehicle is the most you can consider before regarding it as "high milage", simply double that for most diesels. (This absolutely DOES NOT apply to the Ford 6.4 hunk-o-junk. If it lasts as long as a gas motor, you lucked out.)
I had been looking for a 1999-2003 Ford E350 with the 7.3 liter Powerstroke. I ended up finding a nice 2006 Chevy Express 2500 with the 6.6 Duramax with 231,000 miles on it for just $4200. It had been used rather gently by a family run locksmithing company. It never hauled a heavy load and had simply been in-town driving. I live in the Phoenix area,...so no rust issues. I had my mechanic buddy check it out with me for any hidden signs of trouble, and he gave it a good bill of health. These normally sell for about $6000-$8000. It needed some minor attention to the a/c system, but that's it. I'm quite happy. I have seen many of these with over 350,000 miles on the odometer, some over 500,000. I'm confident mine will double its current odometer reading before any serious motor issues. The transmission? Meh,....we'll see.