I'd get a set of Allen wrenches from Sears or your local parts store, metrics, a good temp gun, a big box for everything, especially if you own a pick up truck to slide the box in when you're ready to go. Get some air filter oil, I use cheap 100% mineral oil from the drug store, extra filter foams, tire glue, I use loc-tite superglue gel from Wal-mart, a small container of hand cleaner and rags, a method of hooking up a charger to a cigarette lighter, granola bars and bottled water, and then my regular misc. supplies such as screws, bearings, and whatever else I have extra of that I've bought over the years.

I usually don't try to carry things like spare diff gears, or anything that would take time to fix. If my car goes down that hard, I just try to help out at the track pitting for someone, turn marshaling, or whatever help I can be rather than fighting with my car trying to fix it in a hurry, making my self miserable and a nervous wreck in the process.