I have learned a lot about different machine and frame setups from joining the homequiltingsystems group at
http://groups.yahoo.com . I would definitely check out any machine/frame combo there before purchasing, as there seem to be many ins and outs. You can buy stitch regulators, for example, but not all machines can take them. Some carriages (the part the machine sits on) work better than others. Some frames, as you noted, cannot handle the deep harp machines.
Some miscellaneous things I have learned from the group (check there, as I'm far from understanding all of this yet!)..... You need to subtract at least 6" from the harp measurement to determine how much space is available for quilting. This is because the rolled-up portion of the quilt takes space away from the harp. Also, there are frames that eliminate the need for periodic adjustments due to quilt take-up.
There are quilters on that group who are using every imaginable combination of machine and frame, from very inexpensive set-ups to professional set-ups. Chances are someone there can help you, plus there is a wealth of information in the old posts, files and photos section of the group's website.
Mary