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 basic serger with an overlock function -- requiring just 2, 3, or 4 thread capacity. Note that some models are 3/4 thread machines, meaning they can only sew with three or four threads -- not two or five.
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. However, a good auto-thread serger will do the work for you.
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 much space do you have? If you have limited space (or a limited budget), purchasing separate machines for coverstitching and serging may be impractical. Sergers that can also coverstitch can save space and money, but switching between the two functions can be complex.
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 best 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. It's also worth checking out YouTube and sewing-related websites for tutorials that can help you overcome the learning curve that comes with some sergers, particularly if you're a beginner. Spending time before you spend your money can prevent a lot of buyer's remorse.