Check out the Sportsmobile website (www.sportsmobile.com). They will work on any of the brands you are interested in and specialize in custom installations - you design the van mostly using their modules and they will build anything ranging from a complete customized van to a van with just a few additions. there is also a wealth of B van info on their site. We just sold our Sportsmobile - after 10 years and 110,000 miles, everything is holding together extremely well.
Regarding the make of the van body, I would stay away from the Promaster just because it is a Fiat. Sprinter models can have a longer body than the longest Promasters or Transits. The Nissans are quite short and have little interior space.
