I've played Rocket League for years, don't worry, there is no pay to win aspect to it at all. All of the cars you can unlock with money have hitboxes that are copies of free cars, so it's purely aesthetic. All cars are the same speed, the only difference is hitboxes. There is absolutely no advantage gained by spending money on the game.
The game is ridiculously fun, everyone should give it a shot. It takes time to actually get good at but it's a very rewarding feeling when you start hitting insane aerial shots.