Some serger blades can be sharpened. Most sergers have one blade that's carbide, which is a bugger to sharpen, so we usually don't. The other is just good steel, and can be sharpened on a grinding wheel, straight across and at the correct angle with the needle plate. When in a room full of sewing machine techs, the instructor asked who still sharpened the blades, and I was the only one who raised my hand. The upper carbide blade on the Bernette is $21.90.
The Bernette 5 spool is one that my boss insists is only good as a 3-spool, and not to waste your time with the coverstitch. My husband got me one from the thrift store, not realizing what it was, and we parted it out. So, that's my boss' opinion, for what it's worth. That machine is also made by Juki.
You don't want to use Singer needles in a serger that's not a Singer because the upper shaft is narrower, and will fall out.
Some Babylock sergers take a round top needle instead of a flat-back needle. I can't remember the configuration... I think it was DBx1, but not sure.
One thing I've learned, is that the taller the thread mast is, the better it feeds the thread. More taller, More better!