Pawz, you seem smart and logical. I'm sure you'll make a great choice for a first horse.
Even though horses look more alike than dog breeds, humans have still had their hand in the breeding selection for hundreds of years. Just like in dogs, in the past hundred years or so humans have caused more extreme changes than were seen before in history. Some of these changes are physical, but a lot have to do with selecting for certain temperamental characteristics.
Just like with dogs, knowing what a horse was bred for will help you out a lot in the selection process. I would read a book about breeds and then read between the lines, imagining how the horse's original purpose they were bred for translates into their conformation and temperament.
Arabians were bred to have tremendous endurance. That requires a lot of energy. Many were bred to look good in the halter show ring. This means they have the ability to get snorty and prancy with a minimum of outside stimulation. Even in your barn.
Pleasure QHs are supposed to jog along with so little action the rider can sit in the saddle and not feel uncomfortable.
Thoroughbreds were bred to race and jump. Obviously most of them are high energy horses also.
Draft horses were bred to pull heavy equipment. They will have a lot of stamina, but only at slower paces.
My point is that so many horses people call "crazy" or "wild" are just bred for specific purposes that may or may not mesh with what you have in mind for them to do.
It can be hard to know when buying a horse whether a horse is what they're represented to be. Horses that seem mellow at first glance might be poorly managed and when properly fed and exercised can become quite the handful. Or a horse that appears well trained might have been doing the same thing in that same arena for 5 years. When you take the horse out of his comfort zone it might be a whole different story.
That's why it is helpful to know some basics about a breed so you can know if you can reasonably expect this certain animal to be what he is represented to be, or if he might change when you get him home and start working with him.
With a grain of salt...my friend has a Connemara/Standardbred cross. Both breeds are supposedly mellow and unflappable. This horse is the hottest horse I've ever met. :)