dont know if its true, but I've been told that the pump has a max working temp, above that the seals go or something
Most pumps will survive temps of 60 deg C, but check your pump first to be sure. I'm going to place the pump before the heater anyway, so I'll only ever have cold water going through it.
I will have a bypass valve fitted, so I can re-circulate hot water back to the main tank, but this will just be used on very cold nights. A temp of 25 deg C over 400 litres would be plenty enough to keep the van frost free overnight.
As for running costs,.. I'm told by someone with a similar system approx £1 per day. Gas bottles cost €25 by me, so I expect to get 3 weeks out of a bottle minimum,... probably double that since I won't use the heater on regular cleans. I do get a lot of one off's and irregular commercial stuff where hot would be an advantage to me personally though.
The law in Ireland for carrying a gas cylinder in a vehicle is very basic. Once the cylinder is secured and upright, thats all you need. In the UK, warning stickers must be displayed prominently on the outside of the vehicle to warn emergency services in the event of an accident etc,... and I personally will be putting stickers on my van even though I don't have to,... its just sensible IMO.
As for fitting it,... as has been said,.. water only leaks,.. Gas explodes,.. I'm going to take the safe option and get a pro to do it for me!