The lighter, the better. There are a lot of lightweight machines nowadays that break down at every good opportunity. It is better to go for heavy-duty machines if you want a true loyal friend in your hands. They are more expensive but worth it altogether. You'll only win over a lightweight in the long term by choosing a heavyweight model.
Many producers pride themselves on having a crazy number of stitches in their models. People tend to use no more than two types, however-straight and zigzag. So ensuring these two are present is enough. Having an Automatic buttonhole is also convenient. If you know you're going to have to make buttonholes, make sure your future pur-chase has this amazing characteristic.
infographic by: sewingtopgear.com