If you are looking at purchasing a new horse in the near future, please take these suggestions into consideration when making your decision.
If you are a beginning rider or just getting started in owning a horse, please consider buying a older, seasoned horse. Keep this little saying in your head: "green and green make black and blue." Older horses (10 or older) will generally be more consistent in their personalities and training. This will allow you to gain confidence in the saddle and correctly learn how to cue your horse without having an impatient youngster under you ... teaching you bad habits such as clinching your legs for balance or riding with a heavy hand.
Try not to be biased on a horse because it is not the "color" you want or the perfect size. Consider the horse's training and compare it to your level of experience in the saddle. Remember: you want a partner under you; the color does not make it a safe choice. A lot of beautifully colored horses are basket cases, and are very dangerous to an inexperienced (or even a seasoned rider) because they have been bred for looks and not brains.
Do your research on the breed you are interested in. Some breeds are more sensitive to the rider than others. By this I mean the horse has to be ridden by an extremely balanced rider with knowledge of how to cue and handle the horse under saddle. Certain breeds also have traits that are bred into the lines for generations that may make them hot-headed, flighty, or just the opposite: level-headed and grounded.
Make sure you get all the information about the horse. The more you know, the better. THERE IS NO PERFECT HORSE. Every horse, no matter how well-trained, has a quirk. Some are very mild and insignificant; others can be extreme. Don' t be scared to ask about the behaviors you may be concerned about. A good horseman will always disclose the good and the ugly about the horse if they are truly concerned about your welfare and the horse's.
Be honest with yourself and your abilities to your prospective seller; do not allow them to push you into a situation that could hurt yourself or the horse. Ask them to demo the horse before you ride; make good judgement on what you see with the communication between horse and rider.
With all this in mind, be patient ... your horse is out there. You will know when you have found your perfect partner.