Water, food, shelter and security. For sustainability those are the basic needs.
Water, should be enough to support what you wish the land to do. For instance the fruit trees, possible garden, possible livestock, family and maybe even power. It is really best to have a stream on site as well as a holding pond that does not become stagnant. In other words you widen the stream or build a pond with a branch feeding in and out back to the stream. Or the pond could just be spring fed from underneath. It is fine to have a well or city water. But it is also much better to have a sustainable source if either of those fail.
Food, again what is needed by you and also what is needed by what you have. Trees can pretty well get by without feeding them. The garden really needs some form of organic material to remain productive. The livestock needs to be sustained by the food the land produces. The family needs to be sustained by the land as well. Of coarse you may choose to have the land not provide your chief source of food, but that is an added expense in itself. As is if you choose to have edible animals and choosing to depend on feed bought from a store to sustain them.
Shelter is basic, but can be set up in a way in which it costs less than a standard home to dwell in. There are so many options on this that I really can't even scratch the surface. You can build a home in a way that it is self supportive as well. With thick insulation, solar heat, thermal heat, geo thermal heat and cooling, the list goes on.
Security. If you are further out away from the city, it makes you a little more of a target for thieves. Security is pretty simple. Bars on windows that allow you to exit said window in case of fire. A steel door frame. A metal door. Mushroom pins in the locks to prevent picking or bumping for entry. Maybe even security door. Secure outbuildings to protect your things. And of coarse guns to protect what you have. This is not only for people. In every venture involving fruit, vegetables and animals, most likely you are going to run into a problem with something else wanting to eat them before you have the chance. This also relates to food storage such as root cellars feed cribs and the like. Just keeping what you have from being stolen by man or mouse.
I have two places, but only live at one, the other has a house that is rented out. For the last 7 years they have produced a profit. The first place is paid for. If all goes well the second will be in 1-5 years depending on the economy. A lot of folks jump head first with very little planning. It is better to set all of your goals before hand and make sure the land can meet all of those needs. You also must factor in that 20 years from now those needs may have to change, so it must be flexible for that. Or at least profitable enough to sell and buy another place that will suit your needs then. Depending on what your current needs are, you may want to look into some books on homesteading. Maybe look into alternative energies here and other sites. Perhaps into farming and gardening as well.
