I have a 2004 Conquest with an alternator and heater/defroster kit. I have a hardtop with soft sides and staying warm in sub zero temps is not an issue. The heater kit has two speeds and both are on the unimpressive side, but I am glad I have it for the defrost effect more than anything. The alternator kit is VERY nice to have when running a lot of lights and doing loooong winches.
I think a good alternative is to put a 12 volt fan on the dash to direct the firewall heat to the windshield or wherever needed.
I have a 2010 Avenger 700 (which didn't seem to be that popular over the 750's, but price difference made it worth it) - I don't have a heater and I don't have the larger alternator. I use mine all winter to plow and that includes running the winch up/down a lot, along with the headlights. I've never had an issue starting back up from lack of battery power. I do keep mine on a trickle charger if it's going a month or more without starting. In regards to cabin heat, It's usually warm enough inside the cab within about 15 minutes that I'm down to a sweatshirt and there's been days in the winter when I'm running it hard that it's t-shirt time. Just my experience.
Early model Avenger with Alt kit,with heater,and plow kit,the heater works excellent,but as said not always needed only issue in certain weather conditions high humidity from melted snow fogging windshield that the defroster can not do anything with,unlike your automobile that incorporates the A/C system to remove excess moisture from the cabin.But glad I have the heater.Cheers NCT