Hi,
I recommend these for checking what you're buying:

LemonCheck - New Zealand Car Check, Vehicle Information & Auto History Lemon Checks
http://www.lemoncheck.co.nz/

Red Book ? Car Price Guide
http://www.redbook.co.nz/

U-Save Automotive (use Vehicle check section)
http://www.usaveauto.co.uk/

And I guess this for knowing what the law says:

Land Transport Safety Authority - about using cars in NZ (like driving licenses)
See ?Visitors & new residents?
http://www.ltsa.govt.nz/
Cheers

Steve