Try joining the homequiltingsystems group at Yahoo! They have a file listing sewing machines and frames for sale from quilters who are upgrading. They also have a file that lists which machine brands are available in each size. (I think 13-15" would be considered midarm.) Once you narrow down your choices, you can Google websites for cost info and search old posts for comments. You can also post and ask for suggestions, although the list owners like people to grub through the files for basic information first.
Edit: Okay, I went and checked their file. Bailey is listed as available in 13 and 15"; I know Bailey is a "stretched" regular machine. Comments on the list about Bailey have generally been very positive. Other brands: Handiquilter is 16", Gammill, B-Line, ABM, Innova have 18". I remember seeing comments from a couple of quilters who recently bought the Innova because they thought it was good value for cost.
Here's a link to the Bailey 13" with cost info:
http://www.baileyssewingcenter.com/homequilter.html