It is up to you to determine whether or not a particular horse is appropriate for you.
What constitutes “appropriate”? There are two components. First, you have to be honest with yourself as to your own abilities, limitations and expectations. Second, you have to be at least a little knowledgeable about horses, or have someone you can trust who is.
The Seller is not under an obligation to assist you. The only real rule is that the Seller cannot lie about something in response to a direct question. This is what is meant by “buyer beware”.
If there are any questions about the horse’s health you should have a vet check done. It is up to you as the Buyer to determine questions about the horse’s health. Do not expect the Seller to disclose it voluntarily.
However, the main thing is figuring out what you want to do with the horse. Are you looking for a pet or an athlete? A companion or a service animal? Do you want to ride or drive? Do you want to compete or trail ride? Of course, these are all good reasons for getting a horse.
If you want to compete, the appropriate horse is going to be further defined by its training. Are you a hunter, jumper, eventer, dressage rider, barrel racer, reiner, trail rider, endurance rider, cattle cutter, roper, or polo player. Each discipline requires its own special training, and they don’t necessarily overlap.
The more particular you are about a discipline, the more refined your assessment of the horse has to be and the narrower the definition of “appropriate” will become. Of course, if you have a discipline, then chances are good you also have a coach. Your coach is the best person to assist you in finding a horse that is appropriate for you.