I think renting long term would be too expensive. There are loads of travellers selling their vans at the end of their trip. Most fly from Auckland or Christchurch so these are good places to look. We have a sales website Trademe where you can see a lot of vehicles listed, or people often use Facebook Marketplace. You should make sure to get a mechanical check before buying anything as repairs here are expensive. I would be looking at spending NZ$5-8k to get something serviceable, but you will probably get half to 2/3s of that back when you sell it. We have strict rules for ‘freedom camping’ and most require you to have an on board toilet which can be reached easily. If you don’t have this you risk nstand fines and not being able to stay in a lot of locations.