1. If you're getting a cutaway van bus, gas isn't awful. They only weight a little more than a regular van, so you're not really punishing them. If you're getting a medium duty bus, most of us wouldn't consider a gas engine. They don't have long service life and they get awful MPG. They're also pretty uncommon, because nobody wanted them!
2. Air brakes are overall better, but the hydraulic brakes in my Vista are amazing. My TC2000 has air brakes and they work well but don't have the feel of hydraulic. For a big bus, air is the only reasonable way to go. Also, you can tap into your air system to run air tools!
3. That's what I'm doing, except my TC2000 is 32 ft. You'll either want a front engine bus so you have the nice back door, or a rear engine with a wheelchair lift next to the back of the bus that opens on the side.