Keep age in mind, I would look between 8-14, but you should be focusing on training and experience.
You want a horse that has been ridden regularly for multiple years in a range of situations. You want a horse that has the correct training. Age is often used as a guide because something under five won't usually have the right training and experience for a first horse or novice owner, simply because there isn't enough time. They are also still learning.
So I guess I'd be asking for a horse that has been ridden regularly for the past couple of years at least, ideally one that has been to riding club or competitions. Ask for proof, if they have trainer receipts, or a trainer who can give you a reference, results from competitions etc.
Get a horse doing what you want it to do now. So don't get one that's been out of work for a few years, or needs a little more training or has a few issues. Get one that is being ridden western pleasure style and does do trails.
A good horse, that's quiet, experienced and trained doesn't usually go cheap. Price range depends on area, so I wouldn't have a clue for your region. If I were you I'd go online to a few places and start look at ads that have horses similar to what you want and see what the going price seems to be. Look for the best horse, not the best deal.
A second opinion, like that of an instructor, will be valuable.