Try out as many as you can before you decide what to buy. I tested several and found THE one that worked for me. Nolting.
I like that mine has the stitch regulator. My 12" frame also has hydraulics so we can use it for any height operator or use it standing or sitting.
Determine what your needs are, what space you have available and your budget.
As for used ones don't be afraid to buy one used as long as it works.
If you go the Nolting.com site they have used ones for sale as well as other used models for sale.
I also like the computer system (SideSaddle) I have with mine.
Good luck.