Personally I wouldn't buy a Singer serger...they don't have great reviews. I've owned a couple Whites (one very old one that was excellent!), a Brother (1034), and now have a Toyota & a Euro Pro, neither of which I'm real happy with. The Brother was by far my favorite & can be found online around $200. Super easy to thread & use, quiet & smooth, good value for the money. I've owned Janome machines but not their sergers, if I had the $ to blow I'd get their coverstitch machine in a second though!
Oh, must-have features (gotta learn to read better). Differential feed, 3 or 4 thread option (minimum), extra feet & plate, ease of threading, especially that lower looper, ability to buy parts like extra cutting blades, off the top of my head anyway...My oldest White would serge through several layers of denim without a hitch, nice metal machine. The Brother was good for that too.