An 18" throat is more than enough space to roll up a quilt in. I use a 9" and have no trouble rolling it up. The problem comes when trying to quilt blocks but then you just have to figure out a way to break the block down into smaller units that look like one larger unit when finished. With an 18" throat, that would rarely if ever be a problem again.
I think features & price are the most important things when looking for home quilting system. The bigger more industrial machines usually come with some kind of training, camp and/or setup but they are out of my budget for now.
I'm a self-teacher kind of personality anyway so training isn't that important to me. However, if I was buying one of the $10,000+ systems; I'd definitely want a thorough training session because I'd be afraid of breaking something.
Definitely try to attend a larger quilt show and get your hands on as many machines in your price range that you can. Setting your top price will eliminate some machines. Making a list of must have features will eliminate others.
Beverly