I got it when it was free for a week or so, it's a great game. Absolutely zero map is needed to unlock all the cars, just slowly play your way through the careers. Online multiplayer is also great, I play with my brother all the time, in the same map, with 4 others.
No, there is no "grinding" for anything. Or repeating tracks. Neither is it very beneficial. In the later stages of careers though, some tracks can get very tricky, but careers alone allows you to unlock some of the fastest cars.