You can use the Garmins for motorcycling but unfortunately I was not able to find out the speeds that they were accurate at. If you are using them at very high speeds there may be an issue and it is worth looking into.

The base maps on the device are basically just land mass and don't include any real detail for navigation. You would need to either purchase a map disc separately or purchase the Garmin 800 with navigation maps. These maps would include the road mapping so you can navigate turn by turn. You can upload routes from map sites, there are various formats supported so I would recommend contacting Garmin or searching their support pages for information on where is best to get these route files.

The cadence and HRM from the edge 500 should be compatible with the edge 800.

It would but it may be a bit hard to read the screen and drive plus there are no voice directioninstructions.

Although the devices are ANT+ and should be compatible with the Edge 800, we can't guarantee that the sensors will pair with the Edge 800 and provide data. We do not stock the Bontrager parts so can't test this.

You will Australia on the base map but the basemap is very basic so you couldn't navigate by it. You would need to purchase additional maps for road or trail for your local area.

Unfortunately not at this time, you would need to purchase these maps separately.

Yes the edge 705 HRM should work fine with the edge 800.

If it is ant+ compatible. Then yes.

This unit will come with European base mapping. Should you wish to have Australian mapping then this can be purchased as extra, link below;