Get one that is air-threaded (much easier and a time-saver!), has differential feed (makes working with stretchy fabrics easy), and I would recommend getting a 5-thread serger so you have the cover stitch (hem finishing stitch that looks like double-needle sewing; check the hem of a t-shirt to see it).
I have an older 4-thread serger without air-threading, and I think I would have used it a lot more over the years if I had gotten the air-threading and the cover stitch. The cover stitch can be used decoratively on quilts.