Welcome - - -
I am fond of my sergers - but they are all older - probably 1990 and older - so - not much help about what is currently available.
I do use mine for finishing edges and for overcasting the raw edges of fabric before it gets washed.
Could I live without one? Sure - but I prefer not to.
I used mine a lot to finish the inside seams of bags - even though they were not "showing" - i thought the finished edges were easier to handle than the unfinished edges were.
The usual advice - what kinds of sewing do you do?
Do test runs on different models.
Things that are important to me:'
Ease of threading - including the needles
How easy to understand is the instruction manual
Is it a recognized brand
How finicky is the tension adjustment
Are lessons/classes available
I have purchased used ones for as little as $35 - and as much as $250 - but I am not a newbie to sergers. I think my Husqvarna 905 might be my favorite one right now.
Strangely enough, I learned more about my "simpler" machines while I was trying to learn how to use the Singer 5-thread serger.