Hi Tom,
I retrofitted a new Espar gas heater into my Roadster a number of years ago. I am able to buy direct from the distributor at a good price and so I've sold a number of kits to other Speedster owners as well. As a result, I have some install photos and experience I can share if you decide to go ahead. You'll find a few photos in my file.
To answer your general questions, the heater mounts up under the dash. The kit comes with most of the parts you need including fuel lines and T's, inlet and exhaust pipes, complete wiring harness with fuses and connectors, etc. The additional items you might need would be a thermostat, and Y diverter so you can select between heat and defrost. I also put a heat protector around the exhaust pipe, and wrapped it with exhaust wrap.
You can find used heaters on ebay and other places, but I think this is an area where going new is worth the investment. Not just because of the electronic ignition and protection circuits, but because they are a bit complicated and if someone is selling a newer unit, it's probably because there is something wrong with it.
