I use an inexpensive brand three/four thread serger (Simplicity) which I purchased new several years ago. The video and the instructions are excellent. It works just fine and I use the heck out of it. It will not do a chain stitch, you need a Five thread serger to do that. However, you can also pick up the old Touch & Sew Singer machines any model number in the 600 series and they do chainstitching when you put the bobbin cover in, you might need to purchase that separately but whole boxes of attachments are sold for three dollars.
You can spend from $300 up to the thousands for sergers so it depends on your needs. My cheapie is great for me.
I wouldn't purchase a Gemsy, the ones on Ebay, tho.
