Good article, but as a horse owner I know that even asking the right questions to a seller can still leave you in the dark. Not every seller is truthful, and horses can be drugged before you come to see them so that they behave and act mellow, even if they are not normally mellow horses. It is always best to ride a horse before purchasing, even bringing more than one knowledgeable horse person with you to ride as well. And a vet check before purchase is always a must.
It takes a lot of knowledge, instinct, and a good eye for horses to know if you are making a good horse purchase. It is better if you can go through someone you know who has horses and has horse contacts who are trustworthy. Even then you just don't really know all there is to know about a horse until you own the horse yourself. I learned the hard way even when I used personal contacts. I was tossed into a wall by a horse I purchased. The seller knew all along that the horse was unpredictable, and she lied to us all. It was a tough lesson to learn when I was a new rider at the time.