Some features to look at are-
stich regulator
12 foot table so you can do King size quilts and smaller
atleast 22 inch troath opening and a good hight so you can do 12 inch blocks on point
I am short $ but else I would get a ABM Innova 26 inch throat opening on a 12 foot table.
A mid arm machine that has a lot of appeal to me is the Avante. It has an 18 inch opening. I will most likely not settle for a machine with a smaller opening Less tehn 22) as most of my quilts are rather large. (up to 10 feet) Smaller quilts I just do sit down on my Janome 6500.