You have virtually no choices for a used diesel small van in the US. Such vans are popular overseas where diesel is cheaper than gas due to tax differences. In the US, no such tax difference exists, so small diesels have not caught on, although interest is picking up a bit.
Volkswagen sold a few diesel Vanagons many years ago, which would fit the bill. But parts and service would be expensive, and these vans are lethal in collisions, because there is almost no crush space to protect you in frontal crashes. Also there are no airbags, and they all will likely be old/high mileage/worn out etc.
You might be able to find a Dodge Sprinter in your price range. These are a Mercedes import and are quite good. They can in several body styles, so some might be the right size, and some may be too long or have too-high a roof for your purposes.
But expect any used Sprinter to be a cargo version, with just 2 seats and a bare metal cargo area. You could go to a custom van or RV conversion place and add extra seats, carpeting inside etc, to suit your needs.
Ford and GM have sold diesel vans for many years, but these are all full-size industrial-strength models, and are probably too large and cumbersome for your purposes.
Be aware that a diesel van sells used at a premium compared with a gasoline version. Consider just buying a front wheel drive minivan. They are cheap to buy used, with many, many choices available. The dollars you save in purchase price and convenience in finding a good one will pay for a lot of extra gasoline.