I think that 16 inch would be the minimum as any shorter arm would be frustrating. Mine is 18 inch and I have had no problem with any of the designs or pantographs I have tried. I also like the Grace Majestic frame. (That comes with the Bailey). I have a Baby Lock Crown Jewel (Same as HQ Avante) which is pricier than the Bailey but not as much as a Gammill. Haven't seen one used, but if you could find one, it is a terrific machine. Some of the new machines can be purchased with 0% financing.
A huge help is to attend a quilt show where you can try several different setups. It is important, though to find a dealer not too far from you.
I think the biggest help for me was the 4 day class offered by Baby Lock (8AM to 8PM). When I was done, I felt confident and was able to start right in to more difficult quilting.