My typical Kickstarter experience has been that after the kickstarter ends, Kickstarter will charge you for the amount you've pledged. Then a week goes by to track down credit card errors, and get those fixed. Some time after that (typically a few weeks, but it depends on how long it takes to get set up and smoke tested) you'll get a email to a pledge manager web site (backerkit) that will give you credit for what you pledged on Kickstarter, and allow you specify which add-ons you want the extra funds to go to, and/or add more funds for more add-ons and also pay for shipping. Some time after that the pledge manager will close and you'll be charged for the shipping and add-ons. Some (long) time after that, your packages will show up. There may be a round of "last change to change your mailing address" emails before things ship.
So:
1. Right now all you can do is add the $80 to your pledge, you'll say what it's for later in the pledge manager.
2. Stretch goals benefit everyone. You can wait slightly longer to pay for add-ons and shipping when the pledge manager goes live, but you'll still end up paying before it ships. I typically don't add funds for shipping until the pledge manager calculates it for me, so I don't make a mistake. SJ Games has always delivered, so I'm not worried about risk there.
3. If you want more dashboards for more cars, you also need tokens and cards to make/play those cars, so discussion on the Kickstarter comments suggested that another core set makes the most sense cost-wise for both sides (to avoid extra labor/packaging costs). More than 4 players is not officially supported and will take a longer time to play, and a 36x36 arena might be too crowded, etc.
See the 2nd tab (Unofficial FAQ) where I pulled out all the replies from the Kickstarter comments - there's a section about games with more than 4 players:
https://www.google.com/url?q=https:/...it?usp=sharing
4. I don't know. They've said "Go on and tell us what you want" and they want new ideas to consider. People are back at work on Monday.
5. If you ask on the Kickstarter comments and someone replies, you can configure your notification settings so you'll see it (e.g. via email) -
https://www.kickstarter.com/settings/notifications