Budget : Setting a budget out front can be beneficial if you have budget constraints. Having a budget to work on, you can easily adjust the types of material to be used or length of wooden planks to be purchased accordingly before ordering the materials.
Another problem you might face if you buy your kitchen island as a separate unit, and not as part of the overall kitchen, is matching it to your existing cabinets. This might not be as easy as your think, but if you purchase a solid color, such as black, white or red, then you should get away with it.
For example, we have seen islands of concrete when the rest of the kitchen countertop is made of granite or another material. We have seen a black island used as a nice "dividing wall" to keep the kids away from a white kitchen.
Plan the height of your cabinet appropriately. Since most islands will have at least two stools or chairs pulled up to the surface, you may want to think about the seating or stools first. There are two kinds of bar stools with one being much higher than the other. Unless the island will be used primarily for family eating, there really are no hard and fast rules on height. It does not have to match the height of the kitchen cabinet, but often does. If it is not going to match it should be taller in most cases.
traditional kitchen islands
traditional kitchen island stools
traditional white kitchen with island