In my opinion, you need to set your budget first. That will narrow down the range of machines to consider.
I have a Bernina 1230 that I love; some of the newer Bernina's I would not like as much. Bernina's are pricey, though; you don't need a Bernina to quilt! You need a machine that you enjoy using.
You really want the needle-down feature, and at least a few different needle positions (to be sure you can adjust for a scant 1/4-inch). You need infinitely adjustable stitch width and stitch length (so you can do different types of machine applique). You will want a walking foot and a darning foot.
Bring a quilt sandwich and some fabrics with you to test drive machines.
The first thing that always strikes me is how noisy the machine is; I like machines that hum or purr, not ones that clink and clank! Make sure the machine sews a nice 1/4-inch seam on the fabrics *you* bring in. You don't want the feed dogs pulling your fabric to one side or the other, you don't want crooked stitches, and you want the stitch to look nice on both the top and the bottom. You also want to be sure you can adjust the needle so that your fabric covers the feed dogs while you are sewing a 1/4-inch seam. Run flimsy fabrics through and a couple layers of denim; the stitches should look good on both without having to adjust the tension. (Having a machine that does not maintain good tension while you sew is one of the worst problems you can have!) Check the wide, long zigzag stitch on both top and bottom; the zigzag is the best way to check that the tensions are balanced, as it will show up any imperfections.
Not all machines make good stitches on a quilt sandwich. If you already know how to free-motion quilt, try that out on the machine to make sure that the stitches look good top and bottom. A walking foot made especially for the machine often works better than a generic walking foot.
Make sure that the machine is full size. You need enough space under the arm to fit a quilt.
Most of the bells and whistles on newer machines are completely unnecessary for quilting. My Bernina has more decorative stitches on it than I will ever want to use. You do want a good blind-hem stitch (the stitch used for hemming curtains) and the ability to mirror-image it so you can do invisible machine applique later; however, even this stitch is not absolutely necessary because you can also do machine applique with just a zigzag.
My advice is not to spend too much money on this first machine. Develop your skills, take classes, see what other quilters are using, try out machines at dealerships whenever you have an hour to spare, and take your time before investing in an expensive machine. It's a good idea to have a backup machine anyway.