I have a Baby Lock Crown Jewel with a 10 foot Grace Majestic frame. A 10 foot frame will not do a king size quilt. You would need a 12 foot frame for that. The Crown Jewel is wonderful - it is the same machine as the Handi Quilter Avante. I do not have the computer but may get it later. I am having too much fun with free motion, pantographs and rulers.
The 2 most important things, I think are to consider what you would use your longarm for and then try out lots. Go to a large quilt show and try them all out.
And second, consider the space you have to put your frame. You have to have space to go around to the back on both sides and to work from the back. That means 16 or more feet by about 9 feet for a 12 foot frame.
You also need to have a nearby dealer who can service your machine.
Best of luck to you. I hope you find what will work for you and enjoy it!