They are not selectable, because you do not have enough money and experience to buy them. You earn that by racing. Series, skill of AI, and number of AI all increase the amount earned, the first mentioned increasing with the highest factor.
Alternately you can go to the files that store your experience and money and edit yourself to a couple of bazillions and buy the entire lot of them.
Go into the rFactor\UserData\"your name" folder, and open the appropriate cch file in notepad... like OWChallenge05.cch
The 2 first lines are:
Experience=
Money=
It's only after we've lost everything, that we are free to do anything.