The smaller and cheaper the camera, the less detail it will record and may prove useless for numberplate recognition.
You need a fast lens and a device to record a fast bit rate.
Dogcam sports have many different ones on their site, have a look.
I favour the vio pov.hd which gives 1080p25 HD pictures with a 140deg view angle.
A bit wide for seeing things the other side of the junction but it does get most things in. It is a 2 part waterproof device, the camera head on the helmet and the record unit can be placed in a back pocket or in a harness. There is a mike on the cable.
There is also radio remote which I place on my handlebars.
One nice feature is that you can set the recorder into loop mode, so it is always recording but only actually keeps the media when you press the remote. ( ie for a 30" loop ( defineable 10sec to 5 min?)) when you press the tag it records the 30 that is being recorded now, the previous 30sec, and the next 30 seconds, a total of 90 seconds about you "incident")
Most motoirst do not "see" the camera, more often, not me either, despite my 6ft2 frame and yellow jacket! Any that do are more likely to "behave" themselves if they do see that I am recoding.
Pull along side a driver on the phone and point at your camera and see the response :-)
I really must get around to editng some of the footage together!