Anectdata: I have over 2000 hours of Rocket League, never once spent any money on it after purchasing it and never one have I felt that my fun was impacted by the available microtransactions.
Nothing that can be purchased in the game affects gameplay, it's all just cosmetics.
It seems to be the most common item people buy (because it can actually slightly affect your performance as it better matches the hitbox, and it is the most used competitively). I know a lot of people who never bought a single item, except for the fennec.
The fennec hitbox is the same as the Octane, but the design of the car better matches the hitbox compared to the Octane. So you can be more precise with your touches.
Nothing that can be purchased in the game affects gameplay, it's all just cosmetics.