I do think you can "overthink " the cars!
But the way I saw it, so long as your kid isn't the one whose car doesn't make it all the way down the track you have done your job!
PH- The cost vary from pack to pack, our "dues" were around $75 a year. You buy the shirt, book, and a few patches. The pack buys the "badges"
If you are considering it for the fall look around ebay for used cub scout shirts in the summer, they will be plentiful as kids grow fast, they will be hard to find once school starts and the pack year begins. We bought a large blue shirt for the kids that gets them through 3rd grade usually.