The GPSMAP 64 uses the same mount as the eTrex
https://buy.garmin.com/en-US/US/p/pn...23-00#overview and can mount on the bars or stem.
If you are happy following a track The eTrex would be a good choice. It will do what Garmin calls advanced track navigation. I like using a route better but that's me some people are more than happy with a track. It has good battery life of up to 25 hours, this will vary depending on how much you use the backlight I turn it off in daylight and 50% or less is the dark. The only disadvantages I see compared to the 64 is that the joystick is pushed in for enter and the screen is a little smaller. This can be hard to do while you are riding as it will move side to side and front to back as you hit bumps. Eater of these are not a big problems and with the eTrex having better battery life I would pick it over the 64 for navigating with a track. One thing I like about the eTrex and the GPSMAP 64 is that they use 2 AA batteries so it's easy to carry a spare set or stop at a store and buy more. I use rechargeable NiMH.
You can get free maps from
Free worldwide Garmin maps from OpenStreetMap they will work on the eTrex and GPSMAP 64 and I think they will work on the Edge 800.