It's sort of new so there are still more cars, game modes, and etc. being added. Touge Drift was just released and it's honestly quite a challenge.
If you don't want to spend money on the game, solo battles at Track A is the best way to grind in game cash. I highly suggest you either grind or purchase your way through because multiplayer should be coming out anytime. I also suggest you use tilt instead of the buttons to steer. Once you get used to tilt, it's way easier and more aggressive than the buttons.
Here's a video of gameplay and someone else reviewing it