I've never been in a balloon, but I do a lot of archaeological air photography in light aircraft, usually between 1,500 and 2,000 feet. I find a standard, or very slightly wide angle lens to be the most useful. I fly with three cameras: 2, 35mm SLRs, one with a 50mm lens and one with a 70-200mm zoom, of which I use the first far more. I also take a Pentax 67II, with a 90mm lens. In aircraft the vibration levels are very high, so you need very fast shutter speeds and so fastish film. The balloon won't have the same trouble, but they do swing and rotate and can still be moving quite fast over the ground. I use 400 speed film, colour in the 35mm cameras, usually Kodak Pro Elite colour 400 (I think it is called 400UC in the US) and B&W (Tri-X) in the Pentax. You will probably get away with 200 speed film, but I would still try to keep the shutter at around a 500th. As others have said, you need to use filters. Even if there is no visible haze there will still be a lot of UV between you and the ground, so a UV filter, or better a skylight 1B is vital on the colour. For B&W (and you can get MF B&W processed by others) a yellow filter is best. Orange and red soak up too much light and can make the ground look weird. Lens hoods are vital to avoid flare. Focus is easy, as everything is at infinity and, just for once, auto exposure tends to work well as you don't usually have much sky in shot.
A few tips. Take far more film than you think you'll need, you'll probably still run out (I average over 140 shots per hour airborne). Also make sure you have everything firmly strapped to you. You really don't want cameras, lenses etc falling out and killing those below. I never do change lens in the plane. I just take a camera for each that I need. Take care taking films out of the camera too: if you drop them you won't be able to pick them up again
Enjoy.
David