Is the ride being organised by a cycling club or group of some sort? If so, I'd ask if the group has any old/spare parts they can donate or let you borrow. Most of us will have some tires, tubes, pedals, chains and brake pads that aren't needed. Even older/worn stuff will get someone to the finish line. Beyond that, a set of tools, energy bars, water and towels and you're sorted.
I'd imagine the bulk of your breakdowns will be flat tyres or snapped chains though, and most of your stops will be riders who are struggling.