Must be a low impact equipment. This means it shouldn't put strain on your weight bearing joints.
Should be able to support your weight. Look for the recommended maximum user weight on the machine.
Should allow you to do your exercise in the most comfortable position for you - standing or sitting, depending on how much you can do.
Must help you lose weight in a most efficient way, as well as help improve your heart health and tone you up.
Must be one that puts some fun into your exercising and makes you want to come back on it at least half an hour every day.