I think you'll always need a gas engine for A/C. A few HP will do it. Not too complicated, and pretty efficient. Might do double duty as a gas heater or that would be separate. Had a gas heater in one of my old air-cooled Bugs. Those sucked quite a bit of gas. And you felt like you had a ready bomb in front of you. Bigger problem for cold climates than warm I think. Looked at a Prius forum and the Canadians said to dress warmly when it gets bitter. One fellow insulated his doors. Seems they would have thought of that. With my Bug that didn't have the gas heater, I put 6" batt insulation on all the floors - and it made a big difference. Still had to scrape ice from the window insides though. And always bundle up. One of realities of efficiency is less waste heat to use.
--Vic