I met a dude that was selling his 1990 Chevy Celebrity. He was bragging how it was so good that he never had to put a water pump in it or starter or other things. Because of that he was asking an inflated price. At those miles, you absolutely will need to put some money into seals, rack and pinion, CV axles, etc. The engine may have quite a few miles to go, but not everything will last 150,000 200,00 miles without repair or replacement. In his case, he should have been selling the car for less than average because soon it was going to need several repairs that will jack up the cost of the car.
The same thing for that Sprinter. Get a list of what has been repaired or replaced. Some sellers hate it when potential buyers ask for proof instead of taking them at their word. Walk away from people like that. If sellers don't want to or can't objectively disclose the condition of their vehicle, it's not worth the risk.
400,000+ miles on a diesel is still a lot of miles. Some will last 600,000 miles, but most won't, especially if it hasn't been maintained perfectly. 300,000 to 400,000 is considered high mileage and usually near the end by most. Of course there are exceptions. No matter what you decide, always have a trusted, competent mechanic check it.