The Zed isn't a particularly good base for a track car in the first place. Too heavy & too many electronics: It was designed as a road car which is where it excels. Sure, you can make it better on track, but an average Caterham is going to walk all over you for about 50% of the cost.
Driver of said Caterham will also be very wet and very cold when it starts raining, but you get the point.