Re: Which are the free cars that you unlock by simply playing the game ?

You only get character's cars (unless there's a car you get at the last event - which I haven't completed), so you can probably find them on Google.
I know that you get Magnus' and Nakai San's 70's 911 Carerra RSR, Ken Block's Mustang RTR and Eddies GT-R (R34).
However, you can't do much (if anything) in terms of mods, so it's still worthwile to buy them so you can tune them the way you like, put better parts etc.