There is no shortage of gizmo's and gimmicks in the health and fitness world.
One thing you don't need to buy is a heart rate monitor. It's true that your work outs do need intensity and heart rate is an important measurement.
To determine if you have reached your appropriate intensity level, use the talk test. If you can talk easily, then you are not working hard enough. If you can only speak three or four words before needing a gulp of air, then you have reached an appropriate level of intensity.