You'll be fine with this rig. Is it a M809? They had the de-rated Cummins NHC250 engine. You would have no problem upgrading the heater, or even installing aftermarket A/C. Spare parts are plentiful, and there are many useful You Tube videos on it. You probably already know that it has air-over-hydraulic (AOH) brakes, and that the front axle engages automatically when there is a difference in wheel speeds. True that they're only good for about 50 mph on the highway, can you live with that? As for the ride, this can be tamed with the installation of an air ride seat. You'll still be money ahead after upgrading. One thing I strongly recommend is installing Dynomat or equivalent sound deadener.
Here is the maintenance manual if you'd care to peruse it: