be careful with some of the machines being suggested because they aren't really designed for frame quilting.
the harp area should be a squared off as possible otherwise you will lose even further throat space due to the curve off the head in the throat space.
also pay attention to the height of the harp area because this is the section where the finished quilt rolls up into and the shorter the harp the less space you will have and large quilts will have to be turned half way thru the quilting process.
you want a machine that is a high speed straight stitch only for frame quilting.