Wow, that's a tough one without a price range or knowing the kind of quilting you plan to do, but I'll try.
#1-machine and dealer service
#2- ditto
#3- ditto
I've got an HQ16 and can stitch a 12" pattern. With robotics (ProStitcher, Qbot, etc), the size is immaterial if the robotics are set right and the stitch regulator is good. On the other hand, without robotics, I can't imagine how you'd stitch very far from the front without hurting your back. And they take a whole lot of room. I'm happy with my 16" and I"d be happy with 18" but I don't think I'd go larger.
A stitch regulator is nice, if it's a good one. If not, it's a pain. I like having an M bobbin. You can make channel locks simply by clamping the carriage, which I did before getting Qbot. Fabric advance would be lovely, but probably way out of my price range. Stylus? Unless you're using the channel boards or something like that (and the styli for those are pretty much particular to the company that makes the boards), I'd think you'd be better off with a laser light, but those are also available as add-ons, so not a big deal.
Are you looking at a commercial machine (Gammill, APQS, A-1, etc.)? If not, the frame should be a real consideration. It's important that it be rigid and solid.
Tell us more about your plans, and folks who know more than I do will probably chime in!
--Heidi
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