There are a just few features that I would be unable to live without. I have a Pfaff (actually three - lol) all purchased used. My most recent is a Creative Vision which I love. Here are my four items that I want on the machine I do the bulk of my sewing on. I would not purchase a machine without these four if I were looking at a new machine on a budget.
1) I want a needle down/up and a presser foot that raises and lowers as necessary - either on a tap of the foot pedal or automatic is fine with me.
2) Dual Feed - I sew all the time with the Pfaff IDT - which is Pfaff's dual feed system. I love it, it works fantastically and I wouldn't piece without it.
3) A single hole throat plate and an interlock system that will not allow you to do a zigzag with that plate installed. Well - there is obviously a story there - I have broken enough needles over the years not engaging the brain before switching stitches and even once threw out the timing on the machine.
4) The ability to move or nudge the needle from left to right when needed (which would require a different plate from #3).
Here are the things that are really nice to have but would not be on a must have list for me.
5) Lots of Decorative stitches - great for embellishment.
6) Free Arm - a necessity for garment construction but for me, I have never been used it in piecing.
7) Top loading bobbin
8) Decent built in button hole
9) Embroidery Unit - I love to play around
10) The ability to add on a large slide on work surface like a sew steady.