I want to say check out the South Island ski fields sooner rather than later (especially if you're a keen skier), as they have a pretty good base of natural snow as well as manmade at the moment...but it will depend entirely on whether you can find somewhere to stay or not. I'm thinking of the Treble Cone, Cardrona, Coronet Peak and Remarkables ski fields. Queenstown and Wanaka are the two main base towns, but you can also look into Hawea and Cromwell for accommodation if needed.
Here are some useful sites with information on buying used cars in NZ:
https://www.nzta.govt.nz/vehicles/choosing-the-right-vehicle/tips-for-buying-a-used-car/
http://www.aa.co.nz/cars/cars-for-sale/how-to-buy-a-used-car/
