Looking forward to seeing how you progress Rob. My car has no heating/ventilation at present so I may go down this route. Which unit did you buy in the end?
.....
I don’t know what engine you’re using, but on my LS the water needs to be constantly flowing, you can’t just block the flow to the core to turn heat on/off, so you may need some form of diverter valve that recirculates the water back to the engine (or not!)