I used this pump http://www.boschmotorsandcontrols.co.uk ... /index.htm
Appears to be often used for installation in cars etc, Also have a three port diverter valve visible to the right of the expansion tank which gives interesting possibilities... I fed this into a basic heat exchanger I got from ebay. Think it was from a Vaillant boiler, with cold water going in it raised it to quite a nice warm temp instantly! Possible on board uses?!
Whilst I was building up my automotive business to the point of sustaining itself, I did a lot of plumbing and heating. This may be obvious, but some things that may help you with your designs, that I learnt from heating systems, and from playing with these heaters.
Place the pump as low as possible, these kind of pumps easily airlock, and don't self prime. Air has to be able to find a natural way to escape, either to somewhere where it can be bled off or into an expansion tank. I had a look around for the best/cheapest kind of expansion tanks I could find but nothing seemed 'perfect'. I may make one out of some alu sheet.
If possible also have the heater self prime. They will cycle about 10 attempts before giving up and need a controller to reset them. I found the exhaust to be really quite noisy and so may use two silencers to help with that. Also the exhaust is really hot, there is a lot of wasted heat. As a little experiment I piped this via another heat exchanger, like modern condensing boilers. This did work but I couldn't assess if it affected the combustion by restricting exhaust flow too much (would cause sooting up) and additionally the heat exchanger would corrode since its not stainless. Then there is the condensate to deal with, and the legalities of it all. So I'm not going to pursue that which is a shame as it would be a whole lot more efficient.
My plan is to build the whole lot into a sealed alu box as compact as possible! Did you get the same type of heater? Would be interested to know what your plans are