Watch your horses' social interactions closely so that you can determine which new horse will go best with which established herd. Your fence only will achieve its best safety rating if it is properly installed. Given the equine propensity for finding trouble, it's crucial to prepare not only for the obvious accidents, but for the less-obvious ones. Place your gates so that horses don't get cornered and crowded when you're leading animals in or out, and fence off swampy areas, trees, power line guy wires, or anything in which a horse could become mired, entangled, or wounded upon. Of course, horses will be far less motivated to leave if all is well and good within their paddock.
Good grazing is the number one concern.
As long as the grass is green beneath their feet, horses won't tend to look elsewhere quite as enthusiastically. Don't forget to provide fresh water and some sort of shelter, as well. If budget concerns limit which fence you can choose for your property, it might be time to prioritize. In the back forty, where you keep a herd of steady, older geldings, you might be able to get away with a less expensive fencing option that would be perfectly adequate and safe for them. When horses lean on your fence, the boards, pipes, or wire will be pushed against the posts rather than off them.