One of the more important things you will have to make a decision on - and probably need to make it early on in your search - is whether you want a computerized setup or not. It will make stitching quilting designs much easier, but will also increase the cost significantly. Or, if you're comfortable with FMQ on a long arm, maybe you'd rather not have a computerized system.
You might check to see if there are any quilters or quilt shops in your area that rent time on a longarm. This would be a great way for you to test drive a machine or two.