... Ok before all the lens tech weenies jump on me, a good lens will produce better results but the best lens used with a poor technique will produce horrible pictures over a poor lens used correctly.
Panning: A must if the car is moving past you when you are shooting. Here's what I do. Plant your feet in good position parallel to the racing surface or the direction the car is moving. Hold you camera properly left and under lens, right hand on shutter side grip. Pull you arms in tight to your torso and stiff. Now turn only with your torso. Twist toward the direction the car is coming from and pick up the car in you lens. Now twist with the car and softly press the shutter in bursts. I shoot 3 frame burst with my 20D so as to never fill the buffer. Like in golf or baseball follow your twist all the way though.
Metering: Camera in manual. Center Weighted metering. Meter off the gray of the track, take some test shots checking the histogram and adjust as necessary. Now leave it unless the light changes dramatically. (Pit road: Go to AV, set your desired aperture and let the camera do the rest.)
Shutter Speed: To start set it kind of high. Something between 1/500 - 1/1000 depending on light. As you get comfortable with panning slow the shutter down. Try 1/250 and then try slower. The slow the shutter the more motion blur you get from background objects.
Aperture: Set it where it needs to be according to your meter. Obviously opening it up will help you get more shutter speed.
ISO: Up to you but for a good sunny day or over cast day start out at 200. Move up as you need to gain shutter speed.
Focus: Use your center focus point set to AI Servo when shooting the racing actions.
Focusing tip: Set your cameras custom function CF4 to 1. This will move the focus function to the * button on the back of the camera. Why? Well now while in AI Server you can turn focusing on and off by pushing the button. Push and hold while panning with a car. Push and let go while shooting on the pits.
Now to your original question. Lens and Focal Length.
18-55 is going to be too short even if you have a great spot on pit road. Personally I'd suggest something in the 70-200 range for pit road. A wide angle lens is good for some interesting shot angles but most of your driver pics will be done with a telephoto. If I had to choose a second lens for this I'd go with Canons 28-135. It has IS and for the money is a great lens. The shorter range may be more desirable as a walk around in the pits.
Shooting the racing action. You need at least a 70-200 and I often use my 100-400. A fast lens is better, f/2.8 or so but I do just fine at f/5.6 with my 100-400.
Lens sharpness: Well as with anything, the more you spend the better results. I shoot with a 70-200L f/2.8 IS, a 100-400L IS, and a 17-40L f/4.0. I started out with a Sigma 70-300 and the canon 28-135. Now that I have the good glass I'd never go back but you need to buy what you can afford and then learn the limitations of you equipment.
Point is, if you follow the advice above and learn the limitations of your equipment you should get some good results. The more money you can spend the more keepers you’ll get. Some hold true with the bodies. I’ll get more keepers from my 20D then I do with my 300D. If I were to spend more money and by a 1 series body the focusing is so much better I’d get tons more keepers then both my 20D and 300D combined. However, my budget won’t allow me to pick up a 1Ds MkIIn so I have to settle for what my 20D provides. If your budget won’t allow for a 100-400 then you have to settle for what your budget will allow. In ether case, don’t let anyone tell you that you NEED this or that to get acceptable results. Get what you can, except what you get, and as the budget allows get better equipment.
For some reference shots look here:
http://www.pbase.com/rkircher/racing I have most of the lenses used in shot info and the mm used is in the exif data. Look this over and you can get an idea of what I commonly use.
--
Rob Kircher
My Stuff:
http://www.pbase.com/rkircher 
