Glad you've joined us crazy skoolie people.
So I would probably do the same thing on the back wall. In fact, our upper back wall is made in such a way that we would have to cut the metal to remove the inside skin and so we decided we would simply insulate over it and put our finished wall over that.
Now the heater...
It's a bit tricky, because it gets its heat from the engine coolant. You will (probably) find that there are 2 hoses running to the heater core (a radiator type thing). These have to be removed - probably at the point where they attach to the mail coolant hoses in the engine compartment.
You would need to get two hose connectors that fit the size of hose where the current "T"s are found and replace the "T"s with them. When you disconnect the hoses, beware that you want to have a bucket handy to catch the coolant that will come pouring out.
Most likely it won't be a difficult job, relatively speaking. If you have difficulty finding the connections, etc. I would have a mechanic show you.