I would never have anything other than a Babylock with air threading. I've had a Bernina and a Pfaff and threading either one would make you lose your religion! The sergers that gave 4 threads are overlock sewing only. Five and more usually include the cover stitch which is LOVELY if you sew a lot of knit clothing...and I do. It puts a double row of stitching on the right side and serves the raw edge on the wrong side.
My best advice since you know so little about sergers is to visit stores and do demos. When you decide make sure your purchase comes with free classes. Buying used is ok if you're experienced but in your case when something goes wrong or you get stuck, you want to be able to take it in and have things fixed/explained to you. There is a lot more to a serger than there is to a sewing machine. If you buy a serger you will not need, nor want, an edge cutter on your sewing machine. Best of luck in your decision...get the very best you can possibly afford and you will get 25+ years of use out of it.