Sounds like you are in the same place that we were about a year ago. My wife hates to do the research. I love to do the research and figure out how things work. I applaud you for taking an interest in your wife's hobby. Some people don't think that husbands are allowed to talk about quilting or know anything about what goes on with the girl's only hobby. I will tell you that my wife and I have been having a blast together with her renewed interest in quilting and I hope that you and your wife do too.
There are a lot of good tips in the previous posts. Pick a budget to start with, but don't be shocked if the target is a moving one once you figure out what you need plus what features you want in a new machine. Everyone else heard you say sewing machine. Maybe because we just bought a quilting only machine, when you said "sit down quilting machine", were you talking about a midarm? A machine that is used just to put the top, batting and back together?
There is a whole bunch of new terms, that many on this site will help you with. Pick a starting point, it's definitely smart to get your wife's wish list and then start the research. When you have specific question, this is a great site to post them.
The truth is there are many machines that will get you were you want to go. It is also helpful if you can tell us what she is using now and what prompted the new interest in a change. For instance. She has a 20 year old machine that doesn't have an auto threader and she realizes that will make her job much easier. Most new machines have an auto threader, but talk to her about which features are important, as in must haves.
I don't want to make this reply too long. But enjoy the journey and remember, it isn't a race. The machine will last many years, don't be in too big of a hurry to spend your money.