Before buying new machine first of all decide what kind of costumer you are
By which I mean an overlocker is a wonderful tool but far from the "best" and only way to do things
Remember that the most astounding and amazing garments created tend to be done by hand, even now. As in with thread and hand sewing needle. Not only that but trims and fabric were woven by hand....
Check what your own machines are capable of and see how far you can push that.
A quick run down on what I use my machines for:
Industrial overlocker: bodysuits long seams, skirt seams, putting shell to lining for flat lined garments (ie all raw edges but not assembly).
Babylock overlocker: more bodysuits, "rolled" hemming of very fine fabrics, joining shell to lining for flatlining. And making tubes of fabric....
Essentially for seams in stretch fabrics and for very quickly finishing raw edges- for quick jobs.
Sewing machine with stretch stitch: piping for stretch and non stretch fabrics, fiddly curves in stretch fabrics and bodysuits, hemming stretch fabrics.
I usually use the triple stitch for stretch curves and corners in stretch fabrics. There are a few overcasting and overlocking stitches which is a fine alternative to an overlocker. It does not cut the raw edge which is an advantage but it it all sewn with just two threads which is a disadvantage when unpicking.
Hevy duty straight and zig zag stitch: all historical and non stretch fabrics.
I only got my first overlocker a few years back and I use it primarily for stretch and modern costuming only. Since getting them my bodysuit production has just become infinitely easier and faster.
I have an entry level Babylock. Partly chosen as it was half price and comparable to cheaper brands, partly because it is made to be easy to thread!
And a small note: the overlocker/serger is not about quality it is purely about speed. That is what they were developed for. To save time not make better
A hand finished hem and seam allowance is time consuming but can be infinitely tidier. I recently hemmed my Mina skirt which is fully pleated. I used a slip stitch and you cannot see any of them. There is no good way to mimic that with a machine and certainly not with an overlocker
French seams are beautiful for sheer fabrics that have no strain I have used it on my Lizzy dress and for a chemise. I prefer flat felling for chemises though.
China/Hong Kong/Parisian hems use a fine and narrow silk binding on raw edges and looks amazing inside flat lined garments.
Flat felling is extremely strong and easily the best for chemises, jeans and any hard wearing garment.
VintageSewing.info
Amazing site with lots of techniques.
Also look on sewing machine sites as they often give examples of what stitches can be used for
And yes, refurbished is love
Though older pre-computorised machines are easy peasy to fix yourself
I know someone who has something like 20 industrial and home machines from the 19th and early 20thC and they are completely amazing. He has restored them himself too.