I completely agree with everyone.
However, I do want to mention that whatever horse you eventually get will be what you make it. My previous trainer liked to say that if you went in with a mindset of "will she do it?" or "I don't think this is going to work" it most likely won't. But, if you sit up there and say "ok, I'm going to take this horse out by herself and we're going to have fun (and not give up or get worried about whatever happens)" it most likely will work out fine.
For instance, my mare, when I got her, was basically bonkers. She was the spookiest, most badly stereotypical Arab EVER. She would jump out of her skin at everything and freak out, without stopping, for seemingly no reason. She was basically the last thing I EVER thought could be trail horse (I had no aspirations of doing much trail riding with her at that point, so it was ok). There are even threads on this forum from a year or two ago where people encouraged me to try her on trails and I flat out refused because she really was terrible on trails.
However, you know what? She's basically the best trail horse EVER now. Watching her, you'd have no idea that she used to be a spooky sociopath. She now goes up to things that scare her and she "spooks" (her spooks are really tiny jumps in place) maybe once every 6-7 rides, even in completely new places. We even had a freaking BOMB go off while we were riding towards it and she just jumped a little, stood for a moment assessing things, and kept going forwards. We've also encountered deep (2-3ft deep) water that she willingly crossed, even though she doesn't like water very much.
She's currently an only horse and we go out on trail rides 2-3 times a week, alone, and she's just as steady as ever. Of course, she enjoys going home but she's perfectly willing to go where ever I ask. For instance, today we went on a 2 mile trail ride in the woods with lots of little creeks to cross and we had a great time.
Her only fault is that she doesn't like following other horses. She wants to be in front at all times. However, she's so steady that whenever we ride with other horses she is generally put in front since she is so "seasoned" and she's a great role model for the other horses. But if you had told me two years ago that I would have basically the worlds best trail horse in a few years time, I would have scoffed in your face.
Now, I'm not recommending that you go find the craziest horse ever and turn it into your trail horse, but I am saying that with the right attitude on your part and the right training for the horse, pretty much any horse can be a great trail horse.
Anyway, story time is over. Haha Sorry for the novel!
Good luck in your search!
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.