Try using the search function because we have discussed sergers a lot.
I have a Brother 1043D which is about $200. It doesn't have air threading but, there are videos on line and it comes with a DVD and it doesn't need threading all that often and when it does it isn't as much trouble as the ones from "the olden days". I took a class and the first thing the instructor said was to totally unthread the machine. And then we learned and it was fine. But you really don't need a class.