The need for a job definitely varies from one dog to the next. I know people who came to my Obedience class, whose BC so needed to be worked that she found her own job - spinning in circles! She was overweight, high-strung, temperamental, and would spin for several minutes at a time through out the day. This is just one example of how neurotic some dogs can be without structure.
For Gonzo, schedules are the best thing for him. He gets 1 hour of exercise sometime in the morning hours, and 1 hour of exercise sometime in the evening, every day! I try not to keep it too rigorous as far as the time he gets to exercise so that in case of an emergency or if I'm sick, he won't go nuts without his exercise. Flyball & Agility are great for tiring them out, Gonzo collapses all day after his Flyball practice. You can also work on long-distance recall and home-made jumps every day on your own. Obedience is probably the most essential, because it creates not only an obedient dog but a mentally satisfied dog. You can start working on Obedience no matter what age the dog is (where as dog sports cannot be trained until the dog is done growing), and you can continuously teach 100's of commands and variations so neither of you will get bored. Even if it isn't pursued as a dog sport, probably Gonzo's favorite "job" is frizbee. He will play frizbee longer than he will fetch a ball, and just getting air and catching it is reward enough for him. There are sooo many fun tricks you can teach with a frizbee, and you can do it at home!
I think that, in the case of some one who is unsure what kind of time they have for an active working dog, rescue is the BEST option! There is at least one Border Collie rescue in every area of the US, and they all screen temperaments and activity levels of the dogs to suit your needs. When you're buying a puppy from a breeder, you can only estimate the activity level & drive of that particular dog based on parents, you have no guarantee. When you adopt a dog that is 6 months+, you have a much better idea of the dog's temperament.