That all depends on where he's going to be shooting from.
What kind of racing are you shooting? Drags? NASCAR? Street courses? Do you have track access, or are you shooting from a grandstand or observation area somewhere?
I am by NO means a pro, but I have shot several ALMS races with full credentials, which gets you the photo vest and track access. With track access, my 70-200 L works just fine, and among the other photogs that are there, that's a fairly common lens. I have the f4 IS, as that's all I could afford, but plenty of other guys use the f2.8 IS. Sure, longer lenses would be nice for some types of shots, but if you have decent access, you can usually move around and find something that works.
If I couldn't get that kind of access, the types of shots I could get with the 70-200 would be more limited, but not impossible. I will say that with the portions of the endurance races that are at night I would have liked to have the extra stop of the f2.8, but the f4 worked just fine as well, and I came away with some stuff that I'm happy with, especially for where my skill level is right now.
As far as what camera body to use, I just upgraded from a Rebel XSi to a 7D, which should be here on Tuesday. From my research, the autofocus system on the 7D seems pretty nice for this kind of work.
OP, can you fill us in more on your situation?
I mainly do drag racing, autocross, and street courses. I do not have track access which means most of the time I will be shooting from the observation area or along the outside fences. My situation is this, I got hired to photograph the Local auto show back in january and didn't have too much money to spare so I picked myself up a T3 with the 18-55mm kit lens. It worked great for me until I started to learn more about photography and my understanding of photography grew. The T3 just seems to be limiting in a few ways...I do not know too much about different bodies, or lenses but taking photos at night with the T3 is horrendous. I am not sure if it is the lens I am using or just the body itself.

I am really thinking on either the 70-200 or the 300mm are there any limitations to either? Any drawbacks? . Can anyone recommend a good macro lens for 4-500?


