Hey NewcastleKnight,
Depends on what you want it for - hot showers, or more of an "all round" hot water supply? If it's just hot showers, then (without knowing anything about the engine bay of the 150-series), I'd recommend a heat-exchanger shower system. But it's not really any good as a "I need a cup of tea now" hot water system.
We've got a Glind unit fitted (with the big pump), for much the same reasons as morgue and Peter - almost everything's already attached to the car this way, and I just need a small additional bag with the hoses and collapsible bucket to make it all work. My system's modified a bit from the standard design, but works brilliantly and gives beautifully hot showers within 5 mins of starting the car, with no need to pre-warm the water. We can do 3-4 showers from a 15L bucket no problems, or pump from a creek for long showers. Can't recommend it enough.
Of course, for boiling water for coffee/washing up, we use just a kettle on the stove. If you're after more of a general hot water system I can't help you, other than to suggest the Coleman HWOD units which I've never used but almost bought a couple of years back. People who have them seem to like them, and seem to rate them higher than the similar blue Primus systems.
Good luck!
Matto