I think the webasto web site can help you with some of the questions.. if you are going with a coolant heater, then I would set up a loop where you can include or exclude the engine coolant loops.. ie if you arent going to be driving the bus for a time, you can heat coolant only circulating inside the bus.. this will help. get the heat where you need..
Joeblack5 uses one to heat his similar short bus nicely.. he swapped to computer fans for one of his heater cores as that reduced the electricity required and noise of the fan to heat his bus (Genius!).
the webasto site can give you the number of gallons per hour of fuel the webasto requires as well as the amps it opulls to operate.
I would also add that to me, a smart move is to pipe it in separately to the fuel tank.. and plasce the fuel pickup of the heater higher than that of the engine fuel.. thus you wont run your nbus out of fuel with the heater, and are much more unlikely to pick up any dirt that could possibly clog the nozzle.. make sure you install a fuel filter in the fuel line for that heater.. thats important towards longevity of it..