Glad I'm not the only one who uses their MZ3 as a detailing buggy. Even though they have a surprising amount of room, you're right...it fills up quickly. Missing from the pics because they wouldn't fit: steamer, LGM extractor and bin of 6.5 pads. Flex, GG DA, and G110 were on the backseat floorboards.
I would definitely go with the Ford Transit. It's the perfect size for a mobile detailer, especially one that doesn't haul their own water source.
Found a couple online for a good price but have a salvage title : ( . I don't think anyone insures these and not sure how it would work out as it would be a business vehicle.