You will often hear people say that you can't grow apple trees at more than 500 feet above sea level.
While there is a germ of truth in this - as in many old wives' tales - it is only a germ. I know of an enormously successful commercial orchard that grows at between 800 and 850 feet above sea level and crops heavily every year.
So here are a few, rather random, thoughts about where to put your trees if you are thinking about planting an orchard.
Think hard about "the slope". The worst thing that can happen to your average fruit tree is to be frosted while in bloom. No fruit that year. Period. So the first, and most important tip is to plant your fruit trees on a slope. Don't go silly and try to plant them where only eagles dare, but make it a slope. You do this because cold air is heavy and falls. It comes to rest in valley bottoms and in hollows and dips where it can fall no further and gets trapped. That is where you get late frosts. Oddly, in falling, the cold air drives out the warmer air that was there before and pushes it UP. In so doing it creates what is called an "inversion" layer. On cold nights this generally exists at between 100 and 300 feet above sea level. When fractions of a degree are the difference between crop life and death, the temperature gradient is measurable and can be critical.
Think hard about the wind. One of the major drawbacks of slopes is that parts of them can be really windy. And wind is bad for fruit trees; pollinating insects work hard enough as it is without battling with head winds. Bees are not the most aerodynamic at the best of times.... Therefore only plant an orchard on a south-west facing slope if it is sheltered or if you can plant hedging or a shelter belt to break up the wind. This is important as the prevailing winds tend to be at their strongest at almost exactly the time fruit trees are in bloom. A sheltered north facing slope is preferable to a windy southerly incline.
Think height. Because of frost pocket risks, don't plant too low and recognise that it does get colder as the land gets higher (at a rate of about 1 degree fahrenheit for every 300 feet). This temperature fall off delays the day fruit trees come into flower and so reduces the time available for them to ripen their crop. I have mentioned shelter-belts and these help enormously in raising ambient temperatures, but unless your position is remarkable, don't try to grow fruit at much more than 700 feet above sea level and try to choose varieties that crop (as opposed to flower) early.
Think soil. People are always surprised when they are told that the human body is more than 80% water. Well, given how solid the body is and how juicy a ripe apple or plum can be, it is not surprising to learn that fruit is generally more than 90% water. So while fruit trees like nourishment and can be productive in relatively poor soils, they cannot do well without enough moisture. The key to moisture retention with plants is soil structure. Fruit trees will die if their roots drown in winter, so there must be drainage and their fruit will be undersized and deformed if there is not enough water in summer. If the soil is thin, or if it is heavy clay, be prepared to incorporate masses of well rotted organic matter. Don't just put it in the hole (in fact on heavy clay, don't put any in the hole) either work it in all over the orchard or just spread it as a mulch and let the worms do their thing. Soil that is in good condition can both allow great drainage and hold an astonishing amount of available moisture.
Think protection. Orchards are a magnet for scrumping children, thieving adults, sheep, horses, goats and the like. You can't keep them all out, but not having a fence is asking for trouble. They are also a magnet for wildlife and in this ever industrialising world orchards are now recognised as crucially important safe havens for an enormous range of animals that are under pressure elsewhere. So leave a little room for some brambles, make a log-pile, have a bit of a pond if you can and cut down on those chemicals (of which there is more coming soon).
Sit back and watch your fruit trees grow.