If you just plan to trail ride or participate in local events, you'll need a much different horse from someone who plans to hit the professional barrel racer circuit or wants to show on a national level. Consider your plans together with your experience.
If you expect to be showing your horse, you'll need to make sure you buy an animal from good bloodlines that is registered in the appropriate breed associations. You'll want to join those associations ahead of time, too, both for networking and to learn as much as possible about the breed. Become very familiar with your breed's ideal conformation and common faults.
If you want to do a mix of local evening--small shows, fun competitions, and so forth--and pleasure riding, you don't need to spend as much for a fully registered pureblooded horse. You'll save a lot of money if you consider a half-Arab or half-Percheron.
If you just want a horse for trail rides with the occasional event thrown in for fun, you don't need a purebred or super refined athlete. You're lucky in that you have a wide spectrum of horses to choose from, so you can find the horse whose personality exactly fits yours without worrying too much about looks and breed.
For more specialized plans you may need to adjust your idea of breed. For instance, your Percheron probably wouldn't be very happy in an endurance ride, but your Arabian might live for the sport. Then again, your Arabian would be out of place hitched to a plow, but your Percheron would love the activity. Keep in mind what your favorite breeds were originally bred for. If you don't want to give up the breed, consider adapting your interests to breed-specific activities.
Naturally, no matter what your plans for your horse, you'll want to have the horse examined by a veterinarian before you make a decision about buying.