Ford lists the height of the mid roof cargo at 72" and the passenger at 67.6". I am 5'9" and can stand straight up in a cargo van with the bare metal floor. My head touches the ceiling in the passenger van, which has a finished floor and ceiling. So if you're doing a build out using the cargo van and want to be able to stand upright, take into consideration the thickness of the materials used. Be advised, the mid roof comes in 130" and 148" wheelbase, but the extended length 22' requires the high roof.
Adequate space is a function of layout, not van length. The desired amenities (eg. bathroom, seating, countertop), what you want to store (ie. will it fit in seat, overhead, "garage"), and bed (longitudinal or transverse). In Europe, there are factory built RVs built on vans as short as 18' long, the equivalent of the Promaster 118".
You will not know what works best for you until you gain experience. Rather than do a gradual build out, I suggest doing a simple, quick, cheap buildout so that you can learn what your needs and wants are. Every system (electrical, plumbing, storage, seating, bed) can be done in a simple way. Do further iterations, based on the pain points that develop as you travel.
For example, I knew that I wanted running water. My first plumbing system consisted of a 7 gal water container with a plastic hose connected to a battery powered water dispenser faucet. It was simple, quick (~10 minutes to build) and cheap (less than $50). It worked great, I used this for over a year. But I recently decided I wanted hot water. My plumbing system became more complex and expensive, now includes an electric faucet, separate water pump, and a lot more electrical power.
