Do you really need a heater? I have a 19 and the motor puts off a lot of heat into the cab. Last weekend I was at 9000 feet and it was less than 50 degrees when we started back down and I had to leave the windscreen up (and that is with only a front screen and half doors). As we stopped, I would crack the half doors to let the heat out of the lower half of the cab. I can see a need for a heater if you are in the snow, using it to plow, or it wasn't going to be over freezing on several of your riding days. Otherwise, I don't see the need.
Being fully enclosed, you may need it to keep the windows from fogging, but if not, I'd try it without one first.