Cars aren't a particular problem. If the cars are close to you then a short focal length zoom such as 18-55 will be fine. If they are some distance away, eg your shooting from spectator areas at Silverstone, then you'll most likely need a 300mm or 400mm lens to get the car as the primary object in the viewfinder.
Manual focus is generally the same on all lenses, twist the focus ring one way or the other until you achieve focus, and if its moving you'll have to adjust as it passes. Autofocus does it for you. Most of the time it does a good enough job.
The capabilities of a particular combination is partly the body, partly the lens, and partly the skill and imagination of the operator.