OK, I've installed Two Espar hydronic units, one Toyotomi forced air unit, and one Wallas forced air unit over the years. I have not personally installed any Webasto units.
The Espar units are truck heaters. In the USA you can buy units set up for marine use from Boat Electric in Seattle. I found that Boat Electric's prices were exorbant and they were not so nice to deal with, but that was just me trying to save a few bucks so I might be biased there.
The Espar units I installed were purchased off the net. I did not tell them I was buying one for marine use or they would not sell them to me.
I had to go over their schematics and figure out a relay network to get all the zone thermostats to work correctly and only run the fan on the heat exchanger I wanted. This was not difficult for me, but I work with relays and electrical automation gear as part of my work so your results may differ on that. Make sure you buy the Espar 7 day timer with the unit. This is critical since when your furnas has an issue it will display the fault code. It is also critical because if the furnace has a shutdown fault and it probably will when you first start it up you will need the 7 day timer to reset the furnace. If you don't get one when you buy the furnace you will be buying one soon, so make it easy on yourself.
A big trick with any hydronic unit is to locate a radiator overflow bottle that you have modified to have a through hull in the bottom at the highest point in the system. Make sure you can add fluid to the bottle so don't place it at the top of a cabinet. Also plan on using a pump and a 5 gallon bucket to initially get fluid in your system.
Cant comment on the webasto units except I like that you don't have to lie to get one set up for marine use at a reasonable price.
