medtech,
I just wrote a long post and the computer ate it. Argh.
Nominally, there is minimal preparation needed for the Massey pre-selection semester. Just high-level high school science and math. That said, I'm not taking any chances, and I have been studying on my own for the past year, and I will likely audit a general chemistry class this summer. Also, I took Physics, Statistics and Calculus III in college. The pre-selection semester consists of Bio, Zoology, Chemistry (mostly O-Chem) and Physics, and you must excel at all of these to be accepted into the professional program.
The application to the pre-selection semester is straightforward. You just need your birth certificate or passport, and official transcripts from high school and any other academic institutions you attended (e.g., college). And SAT scores, if they don't appear on your high school transcript. Getting a New Zealand student visa seems to be the more difficult part, frankly, since you need a medical exam and a police certificate and proof that you have a place at the university and have paid tuition. Also, you have to pretty much wrap up your life in the States.
Vet incomes vary widely, I can't say much except that if you want to make lots of money, don't become a vet, become a doctor. 80% of vets go into private practice, although there are a ton of things you can do with the degree. I think the AVMA or VMCAS web site has more info.
Hope this answers your questions,
Laura