Thanks for asking! Good question, what it means is that you can buy a brand new WRC car, change the oil get new tires and you are good to go. You cannot buy parts as that would break the rules and the server wouldn't let you in but the reason why there is no limit for PP, weight an power is that all four WRC cars have different numbers for all that! If I put a limit for say PP then one car would benefit from it but another one would be at disadvantage. Basically, as they are (Modern WRC cars) are already balanced BUT they have different PP, weight and power so limiting them would put some at disadvantage.
Tuning is prohibited because I want us to race with stock WRC cars (minus tires and oil), without this restriction PP could be increased A LOT!