I think if you have any skills working with 1/4" plywood, then the options of doing your own build are almost endless.
The key to designing your own van will be weight. The total weight of your mods and the distribution of weight in the van. Keep under the limits, and balance your loads, and it could be a good project.
There are many, many different layouts for the Promaster/Ducato if you search youtube. Look at the vans available in the UK. On many of those, everything is located inside the van - nothing underneath. So save for a few exterior holes to be drilled, I think you could build a nice van with just woodworking skills.
There is one layout I remember, where all your services were accessed from inside the rear door of the van - cassette toilet, water fill and drain and replaceable gas bottle. No generator, no ac simplifies things considerably.
Sounds fun. Keep us posted!