Whatever you build into it, the van must be save and long lasting. Otherwise you might loose your work and the money you invest in it. We were looking at several concepts on youtube and decided to look for a Ford Transit, a Mercedes Sprinter or a Volkswagen Crafter which has an identical body of the Mercedes Sprinter.
The motor is important. It should have low milage and a regular maintenance. Safety wise for both of us it was important that the driver AND the co-driver had an airbag. Most vans have an airbag for drivers only.
Since vans are work horses you should look for one that has least wear an tear and no rust. These criteria alone already limit the choice you have. At least in Germany it was quite hard to find matches up in the north were as we found more in the middle and in south of Germany.
Last but not least you have to visit the dealer and see the van. Often the pictures were misleading compared to the actual quality. The odor in the cockpit can be quite awkward – one of the no go criteria for us!