How are you planning to use it? Do you want a 3 or 4 thread machine? Do you want to have a rolled serged hem edge capability? How much can you spend? Do you have a good dealer locally? Sometimes they have some great deals on used machines. Are lessons available? Make sure you get an instruction book to go with it.
A hint on rethreading: tie your new thread color to the old on each spool, change the tension to zero, pull the new threads thru, you might have to actually thread the needle as that knot might not go thru the needle, then put your tension right back to where it was, then your ready to serge. The small amount of thread wasted is teenzy compared to the time it takes to rethread the whole thing. This of course if you don't have one of those snazzy new ones that shoot the thread thru with ar.