I have a 7D as well as a 5D MkII and with the 7D the f stop goes down the more you will have in focus. I think the smallest setting is 2.8 on that camera (which the camera is a great choice). You have to also consider shutter speed and make sure that the auto focus is turned off on the lens. I also turn the IS off and use a mono pod or tripod with a remote button. You will get great results using a smaller f stop if you so desire. I would say practice shots will help you if you have the time to get them in prior to the shoot. Using a higher f stop means you have a longer shutter speed which in turn means the more the image is in focus. This is hard to do hand holding the camera. As the number increases the opening of the aperture decreases and so lets less light in. By decreasing the f number the aperture opens up and more light enters the camera. For this reason a higher f number requires a longer shutter speed so that the image is correctly exposed.

There are so many variables to consider which range from type of day (sunny, cloudy, etc.) One car or several. You should keep in mind the distance from the subject, use of lighting and also how much time you have for the shot. If you are shooting for the photo for yourself or the owners. So if you are shooting in hopes of selling to other than the owner make sure that you have a model release form signed by the owner. This is more important than the settings. Outside start with a middle of the spectrum settings. Inside use a larger lens opening and off camera lighting.

Composition, Composition, Try not to cut the ears off. Use the thirds method and also set your aperture a bit larger to get more of the subject in focus. Also your flash should be off camera for a better lighting as you will be able to add light where you need it.

I couldn't agree more with Tripod Tripod Tripod... I usually shoot with ISO around 200 to 100 f-stop should be on a large setting to witch is the smaller number to allow the most light ie 8 and a slow shutter speed also increases the amount of exposure for the shot you are after. These settings vary depending on your subject. If it is moving you have to adjust for that if it is stationary then you can use a slow speed like 30.

You should always cover yourself and get registered with your state and file for an EIN tax number. It makes you legit and it adds weight to you doing business as a professional. If you will be using all the perks as a professional then you should look into adding that. Don't go out there and say you are in business without having the necessary documentation. Also look at all the tax breaks available to you in doing so.