It's a problem GT has never worked out : the first steps of tuning are (or become) useless.
In real racing (track or rally) cars are divided into classes and each of these allows only some modification to the car : for example you can't change the gearbox of a group N rally car.
In the game there's no limitation like this , so why should you lose time with the first steps of upgrade ? After the first races ,when you have only a little money , you can easily buy the best exhaust , turbo , chip , gearbox and transform a little commuter into a racing beast which is always allowed to race.
Race requirement based on power ( like in GT2 )are not even enough if you can freely reduce the weight and change the suspensions and everything else.