Go for something simple, I think you only need New Zealand Grid as well as waypoint function, since you always carry a map and compass so some fancy function is no use for us.
It would have been useful for those poor buggers that got lost on Mt Egmont in whiteout conditions…I recomnend a Garmin etrex or the Garmin 12Xl that I have and found extremely useful. Check out trademe.co.nz as there’s always some on there.