Ant,
I wish I had as much faith in my "expertise" as you.
Judging from the number of 2 x 1" tube radiators being sold for extra-capacity replacements for cars, I'd say that one should be adequate for you. I'm not sure about the 4 x 1/2" tube design. Theoretically, it might be more efficient (and possibly more reliable at high pressures). I can't imagine a 3" tube design being reliable, but what do I know...
P.S.
In the process of checking out NZ climate, I noticed that you've got a lake ( Lake Hauroko) that's 462 m deep! I don't think anything in the US can match that.
