I'll clarify :P
Pro tuner is the highest class. There are no restrictions on anything. Some race cars will be allowed, but you have to tell me first and i'll tell you if you can use it- and it wont be a case of me letting some people and not letting others, its just hard to write up a list of allowed/disallowed cars.
Club is the middle class. You can have up to 450 horsepower, and your car cant weigh less than 1100 kilograms. Racing slick tyres are not allowed in this class. You can use the ballast and engine limiter to keep your car within regulations, but its pointless building a 700 horsepower car only to limit it to 450 anyway.
Street is the lowest class. You can have up to 400 horsepower, and your car cant weigh less than 1250 kilograms. Racing slick tyres are not allowed in this class either. You can use the ballast and engine limited to keep your car within regulations. The only racing parts you can use in this class are suspension and exhaust.
The muscle car class is sorta just there for fun, where there are no restrictions apart from the car being built in the 60s or 70s, and it must be from America- must be a muscle car.
Simpler? More confusing? lol.
edit: and I have a feeling I may have said 'Japanese cars only' or something similar somewhere, but I'd like to open this up to cars from any manufacturer. Cant wait to see someone in a tuned corvette :D
edit: ahh cool, the GTR photos. I'll start a write up now. What do you think of that racing exhaust btw? I wish they'd done like... one that comes out of the left side skirt or a front guard or something, not off centred in the middle of the rear bumper XD