Ahhh, the old 'what lens' chestnut....
It's a difficult choice, based on your ambitions, skill levels and budget.
My advice is always go the 2.8 option. Simply because these lenses are typically of a higher standard. Using 2.8 in motorsports is not all that useful due to DOF issues, but it does give you an extra few strops of light in low light. This isnt so much of an issue with D700 or D3 bodies in the Nikon world, as high ISO (around 3200) still produces usable images. Dont know the specifics in the Canon world.
Focal length wise I would definitely have the 70-200 2.8, and a 24-70 2.8, or there abouts. If the budget is tight, then I recommend the sigma varieties of these. They are super sharp and will produce images on par with 'brand' name versions (in particular the Sigma 70-200).
If you can afford a 300 2.8, then try to pick up either a second hand prime, or go the 120-300 2.8 Sigma. It's probably one of the best lenses (IMHO) for motorsports as it give you super sharp images, quick focus and is as sharp as the primes, with the added bonus of the zoom. The lens rates around the 95% on FM forums.
I would avoid 80-400's or 50-500's as they tend to be slower focussing, especially for head on shots. Thats not to say it's impossible though.
With regards to bodies, if you want to go down the sports path, then FPS becomes important, as do MP. That being said, I have 2x D2h's for back up bodies and they still produce quality images, although I do prefer my D700's!
Depending on what you have budget wise, and if you want to chase the pro dream, then maybe look at purchasing used pro bodies, it's a good way to gain experience with high end bodies, and typically they have better AF, tracking, and FPS rates.
Really, like has been said above, it boils down to your ambition, budget and willingness to learn.