Don't want to disagree with GM but I'd also consider Dog Cam Sport for accessories, their suction mount is extremely sturdy and I've used it to mount the GP externally to the rear and front glass also. Though I would recommend a second teather if you are doing that, for added confidence.
http://www.dogcamsport.co.uk
If you find a good external mike and windsock combo, post it up, Getting decent audio is the bit I've struggled with most, so far
If you are planning on using it for track, remember they don't allow suction type mounts, so you'll need a headrest mount or a semi permanent adhesive mount for the windscreen.