re: APU
Well it would depend on the speed since drag goes up exponentially with it. I'd say, if you kept the vehicle around 60 and it was the weight of a standard compact, the horsepower would probably be 50 or less. If you could reduce weight substantially and ensure the thing was extremely aerodynamic, it would probably be way less.
What's a bummer is that the AC wouldn't be trivial if you think about what kind of power it takes to run even a room sized unit. I often wonder about mileage ratings on vehicles compared to what you truly get in the hot summer with the AC on.
Art