I have a check list, and narrow the choices as to whether the machine meets the check list. My list (for me) is:
1. Able to lower feed dogs.
2. Stop/start button as well as foot control
3. Variable foot pressure
4. Thread cutter
5. Good button hole method
6. Needle up/down function
7. Twin needle sewing
8. Good needle threader
9. Wide harp - 9" or more
That's about it for me - if it has all of those things it is considered.