Hi Pro-Systems,
What would work for you, if you were prepared to travel, is a fitting service for people who have bought all of the bits, but did not have the time or inclination to plumb/wire/bolt it all together.
You could perhaps charge £400 complete fitting service, plus a suitable out of area travelling fee, which should take you about 4-6 hours to carry out. You would need to have good public liability insurance and a template contract for the client to sign before you started cutting holes in their van. If you started fitting tanks for them though you would need to get serious about bolting a proper frame around the chassis, for this you could charge extra.
The trouble with selling the type of system that you have pictured above (apart from the safety issues) is that the type of client who would be happy having such a basic system would probably just build it themselves.