Back in the day, I used to carry hoses, belts, plugs, fuses, fuel pump and filter, etc. These days you can get anything tomorrow from anywhere, so I carry a credit card. :B
There's not much that goes wrong with the E450. Fuel pump is probably the most likely thing to have an issue with, but it's not exactly side of the road DIY so no point in carrying a spare...
Best advice would probably be to make sure everything is in tip-top shape when you leave. Flush the brakes, make sure front end is solid, good tires, and change all fluids. Advice I should be taking right now, actually!
