Buy the best quality machine you can possibly afford. There are three types of sewing machines, mechanical, electronic and computerized. All three form the basic stitch in the same way. A sewing machine needle carries thread down through the fabric, where the thread is caught and looped around the bobbin mechanism, then pulled back up by a take-up lever, while the feed dog moves the fabric into position for the next stitch. This is where the similarity ends.
Mechanical machines are totally rotary controlled.
All functions, such as tension, stitch length and width, require manual adjustments by turning a dial. Electronic machines are slightly more expensive. Some functions can be initiated with push buttons rather than dials. If the machine has more than one speed, it is powered electronically. As more and more innovations were added to sewing machines, manufacturers added motors for various functions.