30mm no lower ime, or you'll have a car that drives baddly and shags front arms every 6 months. If you're looking Zetec you'll want bigger wheels to accomodate the brakes you'll need to slow the car properly. Don't confuse handling with cornering grip.

I lowered my old XR2i 55mm and thought it was great, until I changed for a 35mm kit and the difference was like night and day and would not recomend going lower