"Post offices" tend to only exist in larger towns now. In smaller rural areas "Post offices" have been replaced by "Post shops" - essentially just another service within the store, a post shop may sell petrol, food, books etc. as well. Most would be open 7 days.
The iPhone should work, I do know there are free topo maps you can download (see
http://mobac.sourceforge.net/) and also
http://macmapper.com/mapapp/). The Topo maps for NZ are excellent though I'm not sure if you can transfer a GPX/KML track into them. The big issue I see with an iPhone will be your battery life - you'll need spare batteries for longer sections.
You'll be catching the tail end of spring which usually means a bit windy, and rain in the alps/west. I would expect the Rangitata and Rakaia rivers could be running high in November/December so plan on getting transport around these (you probably would as a section hiker anyway).
For the length of time you've got you're probably best to stick to the South Island (though I'm biased here). My wife Jane (see
http://janeandshane.com) took 53 days for this and she had support and was pushing long days. As a section hiker you should be able to fit in the nicest SI sections.
Shane