I had good long chat with the guys at JCS (SW) in Bristol. They import and convert MPV's into pop top / campers.
They have their own mechanics that can deal with 12 and 240v electrics, engine / gearbox issues and also their own upholsterers.
They have loads of stuff that gets taken from a converted van that can be purchased ( seats ETC).
They import Nissan Elgrands and Toyota Alphards as well as Hiace's and loads of other models..
Give them a call for more details.. Their website can be found here..
JCS (SW) Ltd