Sounds like a practical plan re getting a bike, work etc (you've checked out the work visa situation I guess?) Selling the bike at the end of the trip - unless you get a lower mileage, popular bike you might find that dealers wont want to take it - or at least for what you might call a reasonable price!
Renting is quite expensive - I have a friend who comes over from Australia once a year and he always rents so there are good options available if thats what you decide. It certainly solves the problem of buy/sell. I'd say perhaps rent one for one or two trips (see below) so you dont have the down time while you do other things. All depends on how much riding you'll be doing vs the total time you'll be here.
Now to the good bit - places to go!! The South Island is THE riding experience in NZ in my and many other riders opinions. Dont get me wrong, there are plenty of great roads in the North Island - its just that whatever you do, dont miss the South Island.
As for Must See places - (I'm only talking road bikes here, by the way - you dont say if its road only or dual purpose)
North Is: you can easily do a loop around the whole of the NI in 8 - 10 days. Some days will be longer than others of course, but a good plan is go around the coast in most places, and include a trip inland to Taupo, probably from New Plymouth would be a good day run. There is a road on this route called the Forgotten Highway which is a great ride, twisty and a bit challenging on a big bike.
South Is: again around the outside from top to bottom and back is all awesome. 8 - 10 days will be comfortable. Dont miss Westport to Greymouth, then down the West Coast and through Haast Pass. Can be very wet down there - they get several metres of rain each year
. Wanaka, Te Anau, Milford - all must do's. Cut back inland from down south and come through Tekapo, Fairlie - absolutely stunning scenario - and even take a detour up towards Mt Cook if the weather is good. Oops - I've missed out Arthurs' Pass which cuts across E-W in the middle of the SI - a great ride, easily achieved one way in a day, then back the next.
You'll find most roads reasonable, well signposted, however very few are more than a single lane each way outside of the main cities. But then traffic is pretty light too, certainly by European standards!
Hope that helps! Now go and book your tickets