If you want a fully functioning gps that will still use the Garmin HR and Cadence functions, you are looking at the Oregon 300 or higher series or Dakota 20 series.. You can buy one of these with Garmin NT Navigator for a lot less than a 705.. I use the Oregon 550T and work perfectly as a bike computer..

https://buy.garmin.com/shop/shop.do?cID=145

If you are a data hound then stick to the bike specific Garmins, lots of more useful end of ride data and some of the newer models can be setup with optional power meters..