You did not list it, but if you are starting with a passenger van, my first choice today might be Chevy Express, because the OEM seats are so easy to remove and replace, attaching to pockets in the floor. You can get headroom with a cap. A 3500 passenger van should have the 6.0, and will have about 10,000 pounds towing capacity.
But I was last shopping just before the Transit came out, so have no comparison to that. I ended up buying the E-series, last year of production, because I had some specific needs unrelated to camper conversion, and it filled them. OEM passenger seats were best in class for comfort and strength/safety.
Today if I looked at the Transit, I might like that for a conversion or even a passenger van, but am discouraged by its limitations as a tow vehicle. Cost no object, the first choice is the largest Sprinter, particularly if buying a bare van for conversion. 24-foot high top has the most space to work with. The big Sprinter issue were I am (small town Great Plains) is service availability, but if you are a major urban area you will likely be close to a Freightliner service center.
