If you are looking for a trail horse and are not an experienced rider , get an older, gentle, experienced horse. An experienced trail horse will watch out for you as much as you watch out for it. Don't let anyone tell you that a 15 year old horse is over the hill. If you give it good care, you will still be riding that horse for another 10-12 years at least.
And, no matter how low the asking price, make sure to get a vet check and be there for it while the vet evaluates the horse. I had a beautiful horse I was buying that turned out to have a ruptured muscle. No evidence of it in the gait when walking and running for me, but there on examination. Saved me a lot of money, not just in the price of the horse, but in future vet bills, cost of upkeep on an unrideable horse, and possible injury from a horse falling under me or my kids. I couldn't have honestly sold it for anything but dog meat either since I knew about the injury which the vet wasn't sure would heal well enough to be a good trail horse.
Another thing, don't buy the horse that is saddled and tied waiting for you to arrive to inspect it. My uncle was an old cowboy and stock trader and taught me to never trust a horse trader! You want to watch the owner catch the horse (if they can't catch it you won't be able to either). You want to see if the horse is easy to saddle and bridle, if it is easy to lift and pick its feet, if it leads well or pulls, snaps or nips at the owner, etc. Make sure it trailers - or how will you get it from place to place? It is nice to take your horse to other places to ride with friends or in new locations.