The upgrade shop will only appear at the end of challenge mode sessions, meaning only when you get a game over in challenge mode. All of these like the challenge mode is infinity expansible, meaning there are no limits, that being said there will be a way to reset your upgrade stats, meaning if the player wants to go a million billion miles per hour they can, the down side to this decision is not all players will, which is why reset stats is necessary, along with that their will be a option to reset saves as to allow the player to reset their progress.
This should be obvious but I wanted the player to reach tors a set of goals to get rewards. Health is meant to make it a bit easier. Not only that but make the player last longer as in challenge mode it will start off with a lower max health than it would in arcade mode.
Speed is meant to balance out gas, the more speed you have the more gas you will spend, along with that the more likely you will crash the faster you go which can be a good thing or a bad thing depending on what you are trying to achieve.
This upgrade is purely asstetic, there will be a limit on how many bus's there are but you can customize your bus with tinting the color through Red Green Blue values to your hearts desires.