hi marcs . mine was an empty trailer when i bought it, it was built by lynton trailers in manchester. oiginally pulled by a vw lt 50 .i still have the lt. its a 20ft box trailer 7ft6ins wide.
i built the unit from our local gravediggers tipper truck. i used the mitsubishi as they are narrow and mine is a 7seater. its only just a bit longer than a car, total length 10 mtr ,hieght 3 mtr. i converted the inside 11yrs ago.you have the right idea ,exhibition units do come cheap. mine including the vw was 1650quid at the time a gift. its mot exempt and if you pm me i can fill you on with more details. just nearly finished refitting a truck for a mate now. doing it yourself epect to spend 5-6 grand on bits to convert . could be more but not much cheaper. cheers alan.