Make a list of the things you want the machine to do.
Like needle down. Stay stitch. Embroidery or not and size of hoop.
Thread cutter. Feet included in purchase. Straight stitch or zig zag.
Top bobbin or under machine bobbin.
I would love to have a stitch regulator like bernina offers but they are so expensive.
Size of throat is very important if you quilt your own quilts. Mine is a H. Viking Sapphire 830 and I love the 10 in throat.
Watch out for gimmics that look like a great way to go but dont work well. One example is the Flynn Frame a big waste of money unless all you make are baby quilts.