Ok, you want comfort in the cab at startup. In that case the easiest is to add the electric heater you mentioned earlier. In the sales configurator this item is descibed as "Electric auxiliary heater (air) (1,400 W). Optimal driving comfort, even on the coldest days of the year – at icy temperatures, the electric auxiliary air heater provides more comfort while driving. If the regular heating reaches its limits at extreme sub-zero temperatures, the electric heating element is automatically activated. With 100 percent heating power in less than a minute, it helps bring the cabin to a pleasant temperature in a short time. The extra heating power also helps to defrost frozen windows from frost." and has option code 9M5 ZUH.
Besides this core heater you can also order several combinations of air- and water heaters from Webasto that will come with programmable settings and a remote control. If you feel inclined you can also add a module to control the heater from your phone, but this needs to be done with a Webasto part not offered during the configuration process.
In terms of lowest cost and not a lot of labour I'd choose the option you mentioned in the start post as pictured in post #3.
I am wondering what you will do to the metal bulkhead though, will it stay? What will you do the the rear of the vehicle?
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The option I was talking about in my first reply is the second heat exchanger mounted in the roof section used to heat up the back end of the vehicle, only usable during engine state =on.
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I chose the water heater (as part of the winter pack) as I want to always drive away with a prewarmed engine in the winter.