Like the previous poster, the only thing required is the aluminium steering rack. Not sure what other upgrades/mods you may be talking about.
Im sure alot of mods/upgrades are available, companies will try to get on the gravy train in an effort to make momey. This is a popular car after all. Are they needed? No.
Look at the national results, Neil Cragg places second with some wins with the car with just the aluminium steering rack. He chooses not to even use associated alluminium rear hubs.
Im not sure there is another model which is as competetive which is as box stock.