Hi, Aziz here.
Please take into consideration that I am by no means an expert, I'm just looking into all chances of gaining more Unity experience. I haven't made a full game yet but just learning little things here and there, which naturally makes me a slow worker.
If all the above feels alright for you, you can read on.
Looking into the PDF file the reskinning process itself looks straight forward if you are using the same model types mentioned in the file, but I guess I'd have to add a "choose a car" screen since there will be 5 cars in the game. The document does mention how to create a new level but it doesn't mention explicitly if they are expecting customers to add their own levels.
You can contact me if you happen to be interested.