Can't speak to Singer models. I did buy a used serger from CL a couple of years ago. Did a fair amount of researching beforehand for features. 4 thread or more is better. Differential feed IS important as it allows for more varieties of overcasting stitches, rolled hems (think napkin edges), etc. The model I bought is a brother - sorry don't recall model number specifically but is probably about the same vintage. Parts don't seem to be an issue nor are needles - they take regular machine needles typically. Threading paths are the most difficult things to understand so I would highly recommend making sure you get the manual as well. I bought an online copy that was many generations old so the thread path is almost impossible to read. The thread diagram on the inside cover of the machine is also pretty unreadable due to some water damage. BUT the machine does sew very well. I got it for a good price. Worth the service fee to have the tension discs re-set, the threading done correctly, etc. Also ended up finding a class for about $30 that was invaluable. Lots of books included and the best piece of advice - write down your starting settings before doing ANYTHING. Then, with each project, write down your tension settings for each path as well cuz you'll never remember them from one time to the next.
Good luck!
Last edited by NJ Quilter; 02-12-2014 at 11:12 AM.