I'm going to assume you're going to have to keep your distance.
For that I think a crop sensor body would work best due to the crop factor changing the effective focal length of a lens. You'll get more reach.
If you're shooting most outdoor in the sun or bright conditions a f2.8 lens won't be as beneficial to you in those conditions as it would be in darker conditions (under the lights at night). f2.8 would definitely be the best but you could get by with a smaller max aperture for a while until your bank account allows for the fast lens.
You will need fast focus speeds...for that I think a body such as a Nikon D300 with a great focus system would be needed. Canon's equivalent is the 7D I think (sorry, don't know much about Canon).
Always hold the bodies in the store first and get a feel for them. It really does matter how they feel in your hands.
You will want a higher frame rate and a buffer that can handle it. You'll be shooting in bursts as they come around a turn or down a straight away - (based on where you're positioned) and you'll want to fire off a lot of shots. You don't want your camera lagging behind your need to shoot. I ran into this at an airshow this summer. You just have to stop and wait for the camera to catch up.