If all you want to do is add cadence and heart rate, just look into getting sensors for your Garmin. You can get bundled sensors, like the Wahoo RPM Sensor Bundle (speed and cadence) for $69.99, or the Wahoo BlueSC (combination speed/cadence) for $59.00. The Wahoo Tickr HRM is only $49.99. So for just over $100.00, you should be ready to go. The nice thing about the Wahoo sensors is that they are both Ant+ (used by your Garmin) and BlueTooth LE. In case you want to go to another GPS bike computer that uses BTLE, you will already have the sensors that will work with it. Also, you can use the BTLE mode to connect to your cell phone and cycling app, as a backup system, in case you are having issues with your GPS unit or forget to charge it, as I did with mine on today's ride

However, if you don't need the GPS feature, there are various wired and wireless bike computers (non GPS units) that will track what you need, including cadence. Check out the Cateye, Sigma websites as-well-as some of the online bike stores like Performance, REI and Nashbar. Lots of choices there.