It's gently sloping, but over the course of about 50 metres, the drop is about 2m. The steps in the colorbond is a compelling reason to go with timber in this case then, right?
Having been a fencing contractor for a long time, I can assure you colourbond does not need to be stepped. It can also follow the slope if you know how to do it that is... And looks way better and neater than palings IMHO. Put it up once and you're done...
Colourbond all the way for me...spend too many times ripping out rotten twisted treated pine fences that were only a few years old. Not hard to replace colourbond panels and the colours stay the same only the names of colours change.