Juki made Bernina sergers for about 25 years. They parted ways, about 1 1/2 years ago. So, you can buy what was a Bernina 800DL for $549 on Amazon. It's the Juki MO 114D. And it is a workhorse! I ran a business from home where we serged 4 layers of Jersey knit, and it didn't skip a beat. My old White couldn't do it. If you need a self-threading serger, the Juki MO 1000 is on Amazon for around $1000.
4 things to look for in a serger are differential feed, built-in rolled hem, adjustable presser foot tension and a tall thread mast. I know the tall thread mast may not seem like anything to think about, but it makes a big difference in how the thread feeds into the tensions.
Maxilock thread is the best for sergers! Some threads twist so much that they form loops that have to pop through the tensions. And you can imagine what that does to your stitch quality.
Of course, you could pay $3,500 for a Babylock serger that threads itself and runs 10 spools and does the wave stitch.