Laura, you said you don't actually do that much mountain or off-road riding, but that you want a mtn bike because of the hills. Why not just put sturdy tires and low gears on a regular bike or road bike? I have mtn gears and kevlar tires on my road/touring bike.
I also like wearing my normal shoes to bike and get around on, not special clipless shoes. So I use PowerGrip straps, not clipless, not cages, not clips. You can adjust them snugly and pull up on them when pedaling. Your feet won't slip off the pedals ever.
Just a thought.