I’d start by looking up the state & federal laws on the subject. If you don’t comply with them, that gives the insurance companies an out. I’ve heard that in some states it’s illegal to have seating in the back of a cargo van. Some states require them to be installed by certified professionals, and some states might not care at all.
You didn’t mention, or I missed whether it was a passenger van or a cargo van. A passenger van should already have reinforced spots for seats & belts, while most cargo vans don’t, and they’ll have to be created.
If DIY is allowed, I’d just pull a nice swivel captains chair out of a conversion van in a wrecking yard, along with the matching seat belts & hardware, and install heavy reinforcement under the floor for the seat in case of a crash.
PS: I’d put the new seat by the side door, so you have the space behind the driver for kitchen etc.
"Always avoid expensive solutions to cheap problems." ~ OffGrid