The concensus is to buy from a reputable dealer that offers classes on how to use your machine and contines to help as needed. Figure out what kind of sewing you will be doing, i.e. clothing, home dec, crafts, heirloom, quilting. This will help you figure out the features you need. You could start out with a simple machine and upgrade as needed. Take some samples of the kinds of fabrics that you plan to sew on for test drives.
All machines sew well on the stiff demo cloth.
After you have learned your machine basics, take some sewing classes. Good luck and have fun. Sears has them and they go on sale often, so watch for sales. If you are not new to sewing, check out your local sew and vac dealers, the machines are a bit more expensive, but they are VERY nice. Viking is my personal favorite of the high end models.