I don't have a Zetec so I don't know what gutting the airbox entails, but to be legal you can not modify the airbox. You can only replace the drop-in filter. Up to double- adjustable struts are legal, but the adjustable Koni's that are available for the Contours are a good choice.
For wheels, you want a light wheel. I don't know if the Kosei K-1 Racing are available in a Contour size, but they are light and strong at about $140 a wheel. IMHO, just go to a local auto-x and run. If you decide you like it, get a rule book and see what of mods are needed to be legal in a class.
Good Luck,
Andy Hohl
