As I said in an earlier post, my first choice for this type of machine would probably be Sears. Take a wide assortment of fabrics with you to try out on their machine -- everything from stretchy lingerie fabric to denim. You want a machine that will hold its tension and not require adjustment even if you are moving from one type of fabric to another. Take a quilt sandwich with you too. The machine should be able to make a good-looking stitch on a quilt sandwich, top and underneath, with you only having to make the stitch length longer.
The samples that the store provides you with are usually stiff. Starched/stiff fabrics will always show a better stitch quality. That's why you want to bring your own fabric samples. If you can stitch on a single layer of muslin and then continue on to two layers of denim without having to make tension adjustments, that is a machine that won't drive you crazy with tension adjustments. A stretch stitch foot may be necessary when using the stretch stitch on the machine too.
Incidentally, the best way to examine stitch quality is with a wide zigzag. When technicians make adjustments to a machine's stitch balance, they do that by examining the zigzag stitch. If the zigzag is good, the straight stitch will be good too. Zigzag on all of your different materials and examine the stitch quality. You can clearly see a bad stitch when the tips of the zigzag are either pulling the bobbin thread up or the top thread down.
Pay attention to the sound of the machine also. I don't like machines that make loud "clunky" sounds. A decent machine may be noisier than my Bernina, but it won't actually sound "clunky".
In terms of stitches, these days you really want to have at least one "stretch" stitch (for sewing on stretchy fabrics such as tricot) and a blind hem stitch (for invisible machine applique). Also, if you can get it, you really want the ability to "mirror image" these stitches. For example, the blind hem stitch is great for hemming curtains but you want to be able to reverse its direction in order to do invisible machine applique for quilts. I'm not sure if mirror imaging is available on lower end machines, but you can always ask. Stretch stitches and the blind hem stitch are usually classified as "utility" stitches.
The one decorative stitch I really, really like is the feather stitch. You can do a lot of variations on it just by adjusting stitch width and stitch length. If you Google feather stitch images, you will see what I mean. I am not as fond of other decorative stitches. Scallops, for example, take a lot of thread and lot of time to sew out.
Find out what feet come with the machine. See if they will throw in a walking foot for free. (Walking feet tend to be more expensive than other feet.) A darning foot is good for free motion machine quilting, an open-toe applique foot is helpful for invisible machine applique, and the walking foot is useful for both clothing construction and quilting. A zipper foot is helpful, obviously, for inserting zippers in clothing (although you need a different type of zipper foot for invisible zipper applications).