Just some miscellaneous thoughts on the topics you've asked about...
I've been driving around much of the south island, and thought that the most spectacular part was actually the mountains on the drive between Te Anau and Milford Sound. If there trails aren't icy, both ends of the Routeburn track (especially if one hikes an hour or two in, if I recall correctly) were spectacular.
I found Arthur's Pass to be somewhat underwhelming compared with the west coast glaciers and the southern mountain & fjord areas, but on the way there is a place called Castle Hill Basin with amusing and extremely photogenic huge boulders eroded into weird shapes, with occasional rock climbers practicing. It's just off the road, and works well for a short stop even if you don't have much time there.
I currently use the same D200/18-200VR combo you do, and I expect it will be all you'll want for landscapes. To capture sweeping vistas, using the pano head and stitching together multiple shots should avoid the need for any additional ultra-wide lens. Regarding low light conditions, recently I accidently took a few outdoor images at ISO 1000 (I had forgotten to set it back to 100 after setting it to 1000 for low-light indoor use), and was very surprised that the noise was barely noticeable - not even enough to be worth using a noise-reduction plug-in on it.
Re photographing lake reflections, keep in mind that the polarizer can not only be used to eliminate reflections, but the opposite setting can be used to somewhat enhance reflections.
Re high-contrast conditions, you can take multiple exposures (using a tripod) and use digital blending (tutorial somewhere on this site if you haven't already done that).
Oh, and watch out for the voracious flightless mountain parrots (really!) when you stop your vehicle by the side of the road in the southern mountain areas. They are known for eating car rubber stripping, hiking boots, and most anything else they can get ahold of. We had a pack of them descend on our car in one roadside turnout, with one trying to get at the rubber stripping and another on our car hood trying to peck through the windshield to get at our leather gloves on the dashboard! They are difficult to scare off - we just got rapidly in the car and got out of there. Worse than the worst parts of New York!
Lisa