I would like to travel over both islands and dive a little bit everywhere. if 10c is the lowest it will go that is a nice 7mm wetsuit temperature for me. I am considering doing what you suggest and renting a drysuit to save luggage space.
BTW what kind of tank valves are most common in new zealand operators? Din or Yoke? I have Din on my regs, but I can purchase an adaptor if need be.
You should be fine with Din.