My first and main piece of advice is to do some test drives. Try multiple machines by different manufacturers, go to a large sewing expo if you can. Definitely buy a stitch regulator (don't think new ones even have an option to not get one anymore..), it's generally much cheaper at time of purchase. I would also very much recommend getting a computer driven machine if you can afford it. If you want to use pantographs make sure both your table and your machine is suitable.
My second big piece of advice is to make sure you have room for it. Most machines do not reduce down easily, you need roughly the space of a single car garage, while you can make do you really want the length (10" or 12" is best in my opinion) plus 3 feet on either side, and 3-4 feet behind it, as well as maybe 4-5 feet (minimum) in front.
Throat size I think 16-18" is fine, I usually only do about a foot at a time but it all depends on your style of quilting and what you want to do. I have short little arms and don't think I could do much bigger than what I do. You do want all the throat space you can get though if you use Minky, it bulks up fast.
Be aware that loading a quilt can be a lot more physical effort than you might think at first. You get better over time and practice but it is still demanding, much more than my friend who bought her long arm thought. I spend more time on it than she does -- but I try to do one of my tops and then one of hers. It's a bit of a drive for me to get there, about half an hour+ each way, and typically I load the top one day and then return the next to do the quilting. It is still easier on me than trying to force a queen sized quilt through my little old machine
