I drove around last summer with a loaded HCH I with A/C with no problems in California, including a trip to Las Vegas in 110 degree weather with short trips all over the strip. Keep in mind that you aren't going to get the best mileage with a car full of people and A/C.

The HCH can handle the hills adequately in my experience. Like any car, don't expect the best mileage in this scenario either. Do a search of hill climbing threads. Its been covered several times.