The Challenge car is a fairly heavily modified roadster kit, designed for wheel-towheel professional and amateur racing.
The big differences between the competition focused Challenge kit and the roadster kit are listed below, but many of the parts are identical.
The Challenge kit is sold in only one form, namely the base kit.
This base kit, however, contains many more parts than the base roadster kit.
You can still use some Mustang parts, but not as many as the roadster kit.