I am looking at the 159 EXT length which gives about 13' 4" of cargo length. The bed would use 7 feet of that. So there would be a little over 6 feet left. I could use all of that on the side opposite the slider, but on the door side only about 3 to 4 feet can be used to leave enough room to exit through the door. I'm thinking I could fit a toilet/shower combo and a small sink on the 6 foot side and a cabinet/cooler on the door side. All gear would be stored under the bed in crates or possibly some sort of drawer system. A drop down table could be attached to the wall behind the driver seat to be used when the driver seat is swiveled to the back. I may have to use a portable table on the passenger side. Since the cab floor is several inches higher than the cargo floor, the swivel seats may leave your feet dangling in air when swiveled to the back, so there would probably need to be some sort of portable foot rest.
Sportsmobile.com does custom conversions for Sprinters, Chevy and Ford vans. They have a Promaster page started, with some basic info and pricing but no specific floor plans. I talked to them a few weeks ago and they said they have been waiting for almost 4 months to get their first PM delivery. Once they get it they will take detailed measurements, create some standard floorplans and set up a blank template of the cargo area and furnishing pieces for self designed plans.