Great project. I looks like you are more than capable of turning this thing around.
The wire mess could be from a trailer controller or, since this was a fleet van, a monitoring device.
For your high roof, I’ve been considering doing something like what you have in mind, but I do no thing it’s going to happened. I would consider a few things since you have a blank canvas. With the roof, this thing already provide very little rollover protection. Since you’ll be building an upper framing, build some legs and do a roll cage around the passenger area. If you plan to do floor to roof cabinets, use one of those walls as a structural wall. Lastly, I know it’s a draft, but add some triangles to the horizontal section of your roof.