Is there a place that you can test drive the different machines? Not just a minute or two sales demo --- really put it through it's paces? That would be the best way to tell what you need. I am just starting the research process myself on purchasing a longarm. I already know I want a fourth rail (for batting), ability to change frame size to accomodate crib up to king, a stitch regulator, and a stylus of some sort (haven't decided if it will be a dowel type or a laser or maybe both is they're not that expensive). I'd like to be able to use the machine from both sides of the frame. I have seen a couple that pretty much fit this bill. Have you considered a mid-arm domestic machine (the Viking Megaquilter/Pfaff Grand quilter) and frame might be an option for you, and the cost will certainly be lots less.
I will be watching your thread --- I'm sure there's lots more points to consider that haven't even occurred to me yet. Happy hunting!!