Hi Susan,
Don't know if any one is still looking at this question, but here are some thoughts in case you do look again.
We have spent 2 six week holidays in NZ and have used the rent-a-car plus campgound cabins option in North Island and have rented a motorhome (for 21 days plus each time) in South Island.
We are regular caravanners in the UK and Europe and were reluctant the first time to switch to a motorhome.
There are pros and cons. It would be convenient to have a car for sightseeing and local touring and popping to the shops for milk.
But a caravan is more work when you arrive at a site and it's not so easy to move if you don't like the first pitch you choose....
Also, we have very much enjoyed "wild" camping - i.e. stopping at a remote location (by a lake or in the mountains), where overnight parking is allowed, and some of the places we've taken a motorhome would not be suitable for towing a caravan. Those have usually been our most memorable stops...
At the end of the day it's also a price consideration - you'd need to be very competitive to make it of interest.
We did see a modern Swift Challenger on one site - privately owned - so you won't be alone in owning an English van if you go ahead.
Good luck,
Chris