My advice, don't do it in January... It'll be very hot and unpleasant and heat stroke will become a very real danger and real help is a very long way away. There is a medical center at the rock, but anything more would require a flight with the flying doctors. In fact their website specifically state that heat stroke is pretty much the most common thing they treat.

Our winter months (so May to September) would be my personal pick of dates to travel, much cooler, you won't need to carry as much water with you, and you'll have a much more enjoyable time.