BAD idea to go out and buy an expensive SLR especially Digital SLR. Just like with video, when you add something more to your head get something cheaper. When your starting, you have a much greater chance of bangin it around or slamming it exiting the plane. Once your comfey with the cheap stuff, start adding. By then you'll have a better idea.
For tandem videos, it is not economical go get a digital camera, (NOW, which is not to say that the future is otherwise). After a tandem shoot (the paying kind!!!) you usually hand over the $1 roll of film over to the student for them to develop. Wasting your time and money downloading to disks, or email, or $$$ printing is a waste.
Stay one step behind technology and you'll save a bundle. Stick with Canon (because it's LIGHT), all other camera brands are just peachy except for the weight.
Make sure the motor drive on your auto-focus can keep up with your flying. The Elan, and EOS models are great, some Rebels are OK. This won't matter if you manually focus, but it can limit your flying to focus.
Make sure your lens has ultrasonic technology (Canon, Tamron...etc) so the autofocus can keep up with the motor. Fixed lenses can be slow.
Make sure your bite switch is mono so it continually tries to focus, most new Conceptus Switches are plug and play, you have to special order stereo plugs.
Good luck