You can get a basic hot box - 15lts from exhange engineering for £439 +vat but you then need the control circuit ie something to turn it on, something to sense the need to start like a flow switch, thermostat, fuel tank, chassis if your taking it out of the van. When you build it yourself you don't have a warranty on the hot box as a whole only each componant, you then have to build it, buy a link hose, cable etc etc. Is it worth the hassle ? I've ordered mine from Brayton as they we're recommended when this post was last gone through £1139 all in, it has the Lavor boiler, decent chassis with steel cover not plastic, fuel tank all built in ready to go