If you're starting with zero tools whatsoever, that Nashbar kit doesn't look like a bad way to go. Otherwise, I subscribe to the John Muir school of acquiring tools as your need dictates and your bread allows.

The most likely issue while on a ride is a flat tire. So a spare tube and a way to inflate it (pump, CO2) are essential. Maybe a patch kit if you live in goathead territory. From there, you're talking about issues with drastically decreasing probabilities. With a modern bike, a couple of allen wrenches (cheap) and a spoke wrench (cheap) will take care of anything that wouldn't have you completely bailing out of the ride with a cell phone call.