Great story. I'm always looking for new ideas on how to teach these concepts to my own children and I love this approach. Now, I just need to find a good place for a vending machine...
Vending machines are far from ideal, although they're still plausible if you have a friend who owns a car repair shop or something similar with a few steady workers plus customer traffic.
If I was doing it today, I would take one of the single-serving coffee machines. Some of them have coin slots, pricing controls, and protected inventory. They're a bit expensive to buy individually (a couple grand) but might make sense to share the responsibilities and decisions among a class or a group of family friends.
Because of the smaller footprint, also easy to add to a teacher's lounge, office, etc and it's possible the manufacturer would cut you a break for doing something so PR-able, especially if you turn it into a repeatable school program.
Drop me a note if anyone decides to pursue something like this -- I'd be happy to help where I can: developing the curriculum, yelling at suppliers, or coming up with new lower-cost hardware.
Am looking into how many machines I can get my hands on now...