Honestly, at least with agility, I really do think the answer is to just find a dog/breed that you like and can live with and enjoy, and then, as mentioned above (assuming you are looking for a breeder dog) find the right lines. Obviously there are some exceptions - hounds will often be challenging, some of the toy breeds can be a bit fragile, and you want to look for a generally healthy/sound breed. But really, there is a huge, huge range of breeds that can do well in agility, especially when you are starting out. I expect rally is the same. I don't know enough about flyball to say.
My first agility dog is (at least reported to be) a Mountain Cur - not something anyone in their right mind would throw out as an ideal agility breed. She had plenty of challenges related to her soft personality, but she's been a sound and steady agility dog for 9 years, and has accomplished plenty. More importantly, she's a dog who fits my life beautifully and I adore living with her the 99% of our lives where we aren't doing agility.
There is certainly nothing wrong with looking for a breed that is prone to being successful in sports; if you think you would love living with a border collie or a sheltie or one of the popular breeds, absolutely go for it. They have their own challenges (I ran a BC for the first time a couple of weeks ago, and could barely keep track of where I was sending him), but all dogs are going to.
Even if you spend a lot of time on agility or another sport, you are going to spend more time not doing sports. I consider myself fairly serious, but even on a heavy training week (I have a lesson tomorrow, which is 6 hours of driving and 3 hours of training, plus a few short practice sessions during the week), I'm still spending maybe 156, 157 hours during that week NOT doing agility stuff. Find the right dog for that part of your life, and then just find the right trainer to help you with the sports aspect.