You choose the airfoil by comparing the polar diagrams of the airfoils. The polar diagrams are plots of the coefficent of lift versus the coefficient of drag. They usually show curves for several reynolds numbers. Draw a line from the origin (zero for both coefficients) that is tangent to the curve for the reynolds number in question. Do this for every airfoil you are considering. Pick the airfoil with the steepest slope of the tangent lines you have drawn. That is the airfoil with the best lift to drag ratio. You can find many airfoil polar diagrams at:
http://www.nasg.com/afdb/list-polar-e.phtml
Then the issue becomes one of setting the angle of attack so that the airfoil works at the coefficient of lift that you want it to. This is very difficult to predetermine because you must take into account the direction of air flow around the car body where the airfoil will be located. You also must take into account the induced angle of attack which depends on the aspect ratio of the wing. Rather than try to account for all this beforehand, mount the wing so that you can adjust the angle of incidence. Then run a series of tests to see which angle gives the strongest down force for the highest speed. If the down force is too great for the car's setup, then reduce the chord of the wing while maintaining the angle of incidence until the download fits the car setup. The results won't be perfect but they will be very close. You can come even closer by reducing the angle of incidence a little to allow for any increases in aspect ratio. The induced drag of the wing is half the total drag at the best lift to drag ratio. The induced drag is inversely proportional to the aspect ratio. Therefore, you will get the best results with the highest aspect ratio you can acomodate.
The above proceedure assumes that you don't know beforehand the down load desired. If you do know the download desired at a given speed, then you can calculate the wing area required and determine the aspect ratio beforehand.