Hi and welcome! I bought a mid roof Ford Transit last year.
As far as "must have" I think it depends on what your van build plan is and what your style of traveling would be. For me one "must have" was cruise control, as I noticed some of the Ford Transits don't have it. Many of my road trips involve a few days of lots of miles, and I find it a lot easier to do several hundred miles when I can put the cruise control on.
Originally I thought I wanted a van with back windows, but decided against that as my van build plan changed.
Some things I didn't think of as "must have" but which turned out to be helpful: the factory rubber mat that came on some vans I found really helpful, as together with a carpet, that's all I use for my floor, and it seems plenty flat and comfortable. The black polystyrene side panels that came installed on some vans, I also found quite helpful, as I was able to take them out and use them as templates to cut plywood walls for the van.