There are two Classes in the SuperMini Challenge:Cooper ClassThe Cooper class SuperMinis are targeted as an entry-level motorsport category that aims to limit the cost of entry and of running that car for a season. The cars are quite rapid however and still provide some exhilarating speed due to the nature of the Mini Cooper design. The Cooper Class is open to R50 and R53 Mini Cooper and Mini Cooper S cars produced from 2001 to 2006. There are restrictions on the modifications allowed and the cars must retain many of their original factory components. Upgrades include the braking systems and some of the mechanicals, however the category has a limit on tyres and minimum weight, with a penalty system in place to keep the category racing more even. 
Unlimited ClassIn the Unlimited SuperMini class you will find some incredible Minis, with both BMW Minis and Leyland Minis represented. These cars have few restrictions and owners and drivers are permitted to build and race at higher levels of modification and power and thus the cars can achieve some significantly lower lap times than the Cooper Class cars. These Minis are more likely Sport Sedans and Mini Challenge Cars running on slick tyres.
Are the Cars Reliable
The BMW Mini Cooper S has been in production since 2001, with many tens of thousands of vehicles produced all over the world. Due to the cult status the Mini enjoys, many aftermarket and tuning companies have developed the Mini over the last 20 years, and there are many performance and aftermarket tuning parts for Minis. Many roadgoing Mini Coopers are sporting more than 300HP as regular daily drivers, and some of the SuperMinis have significantly more power than that. Of course, as a race car, regular maintenance and attention to detail are crucial for reliability, however, the cars are a well-proven package when serviced and operated correctly.