One way tickets are very expensive, in some cases costing nearly as much as a return ticket. Most people travelling to NZ will stop off a number of places on the way there and back, even if just to break up the journey. Around now is a good time to buy a ticket. Pick up a copy of "The Mail on Sunday" and it has a big travel section with lots of adverts. A number of the companies who do RTW tickets are very knowledgeable and can help you picking what you want. Do shop around for prices.
You will need a visa to work in NZ -
.
.
http://www.immigration.govt.nz/migrant/stream/work/
.
.
Most good jobs for the NZ summer go early, like now. Some people come back year after year. If you manage to get a job out there, it will probably be little pay for hard work.
You definitely need to be more organised. You cannot just turn up there and find a job. Pop in on Thailand on the way back, anytime from Nov 1st onwards when the weather has settled down.