I would look for the longest, highest harp I could afford. Remember the quilt rolls up into the arm, so your quilting area will become narrower as you go. With a 9" arm, you will probably wind up with a quilting area of 4-5". Many folks do king size quilts with this setup, but you may have to turn the quilt half way thru.
Look for a machine without all the bells and whistles. a straight stitch machine will be good. If your frame came with a stitch regulator, make sure you check to see if the sewing machine you are considering will work with that regulator.
If you have a dollar amount in mind that will help narrow down the suggestions.