^^and class 4 from a reputable brand will work perfect!
and for mounting it on your helmet, use a curved sticky mount, and make sure you do it inside... get a little bit of sandpaper and sand down the spot where you're gonna mount (make sure you only sand right there, otherwise you're gonna see it...
then take some degrease (acetone or something) only use like 1 drop since this stuff will bite through your plastic shell !! and rub the spot clean !
then take the curved mount, remove plastic and put it with the gopro inside onto the helmet (this helps to get it on straight/leveled....
then apply firmly and remove the gopro, now apply pressure on all points to try and get the sticky foam underneath to make 360° contact to the cleaned helmet surface
this is quite hard, you'll probably end up with some edges that wont make 100%contact, but thats inevitable
now let both helmet and sticky mount cure inside at roomtemperature for a good 24hours