This is one of my favorite machines. It hems, understitches, topstitches, and can be decorative. It is commonly found at the hems of knit garments. It can have two or three needles, and the underside is a looper, allowing the seam to stretch. You may also take this machine down to one needle for chainstitching, which is used a lot for topstitching. The good news is that some sergers have a coverstitch function. There are also home coverstitch machines, and I highly recommend the Janome Cover Pro.
This is the bread and butter for sewing with knits. From low- to high-end, it is the machine you use when you need seam stretch and clean finishing. If you are just getting into knits or considering purchasing a serger, a great entry-level machine is the Brother 1034D. It has lots of features, and it is the home serger I have at my studio space.
This is one of my favorite and strongest stitches, typically found on bras, elastics, and even medical garments. It is known for its ability to stretch and recover repeatedly due to the nature of stitch angles. You may see multiple settings for your zigzag stitching on your regular machine. A one-step zigzag is when it is one stitch length per angle. Three-step is where there are three small stitches at each angle. If you do not have access to a serger, this would be your go-to stitch for construction. Zigzag is also used as topstitching and to install elastics.
Depending on your machine options, there may be some stitches available for stretch fabrics. Do test them out with your fabric and stretch to test their durability before sewing a garment.
If you are getting into sewing knits, especially with Spandex, my primary equipment recommendation is a serger. If you haven’t tried one out before, see if there are any classes or open studio times at places around you. We are fortunate in Portland to have Modern Domestic, which offers both.