Not sure if the OP ever decided on what to do.
In any event, I am still a newbie when it comes to photography. I did much as some people suggested, and picked up a rebel with a cheaper 300mm zoom lens. I freaking hated it because I saw it as slow, soft, etc. That camera and lens was sold less than 3 months after purchse. Knowing what I know now I wouldn't recommend any rebel series camera when a good used 7D can be had used from a reliable seller here on the forum for $500 or less.
Now the question becomes what lens to pick up. Not much info was given by the OP and all he ever posted was one post. Simply said wants to take photos of the customer's cars. What kind of photos? On track performance, or more close up photos of body work, engine work, driver in car, etc. I think most people assumed it would be on track shots, but I also know a great many here take amazing pit shots, close ups, and other various shots in and around pits. The two require totally different lenses, though equally amazing shots can come from anywhere at the track. (I've seen shots of cars being unloaded that I wouldn't hesitate framing)
So we can give lots of advice, but until the OP stops back in and lets us know a bit more detail, we were and are all swinging blind.