The Polaris 800 motors can take just about any beating someone can dish out with the exception of a bullet…They may not have the most power in it’s class, but they’re strong and reliable. I have seen Polaris 800 motors get the absolute snot beat out of them and they just keep ticking.
Biggest thing is to make sure the air filtration system is clean and properly sealed. That seems to be the weak link in them. With a dirty or leaky air filter, you will eat your rings relatively fast. There is no exhaust valve seal on those motors, just a seat. Some user experience slight oil consumption, but it’s not really an issue otherwise Polaris would have put a seal there.
I would add full doors and see how it works before dropping the cash on a heater. The cabs of the 800 Rangers become pretty hot on their own, it may be enough for you after the engine warms up.