It's always beneficial to ask someone in the know, your trainer for example, to go look at a potential prospect with you. When I was first looking into buying a horse, I was super wary about the horse being drugged up, so it would look calm but when it wore off the horse would be crazy. But you should always buy from reputable people that don't drug horses for viewing. But I always ask to see a horse being tacked up, see their stall etc so I can see if they have bad barn habits like cribbing, or are girth sour and so on. I also recommend to have someone video you riding so you can compare different horses. I always ask if the horse is up to date on all his shots and has their coggins because you can't trailer a horse without it. I ask about their temperament, habits, if they have any vices or not. Do yourself a favor and always do a PPE. They can be quite expensive if you are planning to do a lot, but I would recommend doing it, even if you are doing the most basic exam, like only flexions. If the vet sees a problem, then I would check further to see if there is a bigger problem that needs to be looked into or just stop there and disregard the horse as a prospect. Never be afraid to ask questions, if the seller refuses to answer or seem defensive, then there most likely is a problem. Remember, the right horse and wrong horse cost the same, but the wrong horse will cost you much more in the future so keep looking and don't settle for less! Good luck and happy horse hunting:)
If you plan to go and try the horse, don't hesitate to try him twice or more. If you try several horses, keep in mind what you are looking for so that you won't want to buy the first horse you try. Ride as many horses as you can, in different places.
If you don't try the horse yourself, ask for different videos, ask about his feed, his habits,..and always ask for vet checks.
You have to be careful with the laws of the country where you buy your horse, and the quarantine that may be required.
Finally, make sure you have someone you trust to help you to choose the right horse. 🙂