If you are a beginner go by any inexpensive used (working) serger. I do have an airthreader babylock but I started out with an old Pfaff, inherited a more advanced Elna later and without practicing on those two machine I would not be able to appreciate what I have now.
Threading a serger is not so difficult if you understand where the four threads go and in which order you have to thread them.
What will you use the serger for?
Granny