I recommend joining the homequiltingsystems group at
http://groups.yahoo.com . They have a lot of info in their files, plus it is a large and active group so there is a lot of information in the posts.
There isn't a single best, affordable brand. It's a lot like buying a car -- different strokes for different folks.
My understanding is that a harp size of 13" to 16" or so is considered a midarm. 18" and larger are longarms. Harp size is the area under the arm of the machine which limits how big a roll the machine can handle. The smaller the harp area, the less area that is available to quilt once the quilt is rolled up. I believe the rule-of-thumb is to allow 7-8" of harp space to accommodate a large quilt. With a 13" harp, that leaves 5 to 6" of quilting area towards the end of a quilt. Many quilters like longarms because they can do 12" designs at the end of a quilt as well as at the beginning.
But, I'm not an expert in this area......
Oh, and I don't know where your initial post was before, but it is now in the Main section of the forum.