I would make sure a serger I was looking at has differential feed, four thread capacity and is easy to thread. There is a brand that has "air threading," I think it is called. Supposed to be super easy to thread and that makes it easier to use.
Edited to add: It's the Babylock brand that has air threading. Here is a link to their site.
http://www.babylock.com/sergers/
Have fun searching for your machine.