Do try out machines at a larger show so you get a chance to compare. While you may not really use all the throat space (mine is 28" and I rarely use more than about 15-16" near the front) get the largest you can--generally the longer the throat space the tall the machine is too and the height is important when rolling up a quilt. I purchased a 2006 Gammill Classic Plus 5 years ago--definitely you want stitch regulator and a solid frame/table where you can do pantographs if you want (usually from the back of the machine). If you think you are going to quilt for others for a business, then definitely consider a computer guided--if you plan to mainly quilt for yourself probably not as important and they really do hike up the price.
make sure it's a solid build from a solid company--these things cost about the same as a nice used car so you want one that has tech service back-up anytime by phone. Features I'd suggest: M bobbins (larger ones); laser light for panto's and motif tracing; ruler base; separate bobbin winder; easy maintenance for owner; adjustable top roller bar so you can check under the top before quilting(Gammill and Innova both have this). One thought--if you are in a guild put the word out that you are looking--sometimes due to health an owner will decide to sell and you can get a good machine and also immediate source for instruction and help.