There are many things to take into consideration when purchasing any equipment used for water sports. Am I using this for competitive or recreational use? Will it become obsolete? Will I be using it alone or with someone else?
When choosing a kayak in particular there are many features to consider. One of the first decisions to make is to decide how the kayak will be used. For example, you will not want to use the same kayak in the ocean as you would use in a pond and could even be life threatening.
If you are using the kayak by yourself you need to know that you can comfortably lift the kayak to the height of the rack you will transport it on as well as put it back in storage. If you are purchasing a two person kayak, the same thing applies. Both of you need to be able to easily move the kayak around as needed.
The next concern is storage. Are you going to store it indoors or outdoors? Do you have enough room for the kayak you want?
The next consideration is what type of kayak you are going to buy. If you are looking for a light kayak just to paddle around in ponds and such a small recreational kayak that is easy to pack will probably do....