I would lean toward a sealed body and lens combination for on the slopes. You can't predict the weather and if your camera can't withstand the weather you miss your shots or could ruin your camera in the process.
But the first thing you need to do is set yourself a budget. Costs will blowout if your not careful.
As for the Tamron 70-200, I would lean away from this as your traveling you will want to pack a small kit. Don't get me wrong it seems to be a fantastic lens, and I love my sigma 70-200, but its just too big for travel.
The things I think are most important after sealed body/lens are:
- A wide angle lens (da15 maybe da12-24 f4) as New Zealand (Queenstown especially) have MASSIVE views. They are incredible, and you will want to capture that in High IQ.
- Spare batteries kept in a pocket close to the body as battery life is halved in the cold.
- Flash might be necessary to get the exposure correct on the subject against snowy backgrounds, Otherwise the subject will turn out dark! (your on-board flash might suffice, but I use a hotshoe flash which has a bit more power)
- A weather protected camera bag just in case the weather turns on you.
- CPL is important to get those punchy blue sky's and vibrant colours. (I would personally just get one for each lens as you don't want to be constantly swapping the step down rings, but that is a personal preference)
- Lots of Silica packs to have stashed in your bag to help protect from moisture. (and when going from cold to warm, I suggest leaving gear in your bag for an hour or two to prevent condensation forming)
- I suggest a small tripod, carbon fiber is better then aluminum as it doesn't freeze your hands as much in the cold, is better for dampening vibrations, and is lighter. The draw back is cost. I personally went for aluminum as for my level of shooting the difference between the two would be negligible and my testosterone mistakenly says "lighter pffft, that's for sissy's give me the heaviest one in the shop"
I have probably missed some of the most important parts but that's all I could think of for now.
Edit: you can pick-up a weather sealed body and lenses second hand for quite reasonable amounts. So i would suggest looking down that path first.