I think that with the price of Grand Prix horses, it's impossible to buy them when they are in the high level.
You need to buy them at the age of 5 years old, the main quality is the locomotion, a good balance. When we jump, we need to feel power, and a lot of balance. The dream is to have a great temper too. When we buy a horse, we often think that we buy the dream horse, but when he arrives at home, we realize he has flaws, and you have to learn how to deal with them!