Do you sew, too? Sergers are specialized machines that are usually used in addition to a sewing machine. Sergers can't baste, install zippers, make buttonholes, sew in reverse or embroider. Don't make the mistake of thinking you can use a serger for all your sewing needs.
Will you just be finishing edges? If so, you don't need anything more complex than a 2/3/4 serger.
Do you have the capability to thread a serger? Those who have manual dexterity issues or poor eyesight may struggle with the sometimes complex task of threading a serger.
Do you want to coverstitch? If so, you need at least five threads, and the machine must specifically say it can coverstitch. You can also buy dedicated coverstitch machines, but these can't trim fabric for a serged edge.
How often will you use the serger? People who serge only occasionally or on light fabrics or easy jobs can easily get by with a basic or less-expensive serger. Those who use their machines a lot or who do a lot of heavy-duty jobs -- like making jeans -- will want to buy the most machine they can afford.
Experts on purchasing a serger can't say it enough: Try before you buy! Most sergers are expensive and they're also a highly individualized purchase -- some reviewers swear their sergers have not only personalities, but good and bad days. Go to a few local dealers and sit down with the serger. Thread and unthread it, sew with it, and familiarize yourself with its various stitches and its quirks. Some dealers even offer classes where you learn how to use a serger, or several, so you're comfortable with a model before you buy it. Spending time before you spend the money can prevent a lot of buyer's remorse.