I'd endorse what vectis says, up to a point. Depends if you want a "campervan" - in the sense of a motorcaravan, with built in cooker, shower, toilet, etc: yes you'd need all the certification he mentions.

However a lot of people do it the other way, i.e. go camping with a van, stay on campsites or wild-camp, sleeping in the vehicle but cooking on portable stoves. Which is what we tend to do. Though wild-camping is getting a bad press in NZ at present and lots of councils are generating bye-laws against it, it can still be done and is done a lot. DoC campsites are cheap, basic facilities, but in conservation areas (generally wild and beautiful). Details are easy to find.

The website below offers lots of advice, as well as having details of vans and cars for sale. Very often you can pick up camping gear as part of the deal

http://www.backpackerboard.co.nz/noticeboard/cars-campervans.php

And for buying guitars, use the trademe.co.nz website while you are in Auckland or just troll the CashConverters shops (there are lots of them) until you find something you like.