Apart from buying the best you can afford for its performance and features, the next thing that I think you should look at is the warranty and customer service. I have had a deal of trouble with lack of customer service with a once reputable company , that I'm thinking of moving to another brand.
The things you should look for in a good quilting machine are these: a wide throat or harp - 9" or bigger, ability to lower the feed dogs, a single stitch needle plate, a dual feed foot or walking foot, free motion quilting foot and a flat bed table. A knee lift is also helpful so that you can keep both hands on the quilt. If you find all of these features in the machine you are off to a good start.