You don't need a huge vehicle or a 4x4 to tour NZ, most vehicles would be fine on a gravel road & a big vehicle may be limiting as scenic roads can be narrow with steep drops.
Compared to the UK, you'll find the choice of vehicle to be more limiting and possibly older depending on your budget.
There are van-sized 4x4 campers made by Mitsubishi, Toyota, Nissan and VW - to name a few, but you're less likely to find European models.
If you're happy to camp, use a long-drop toilet & be fairly self-sufficient, I suggest hiring or buying a car & take a tent. It'll be much cheaper & if you find a car you can sleep in, that's an added bonus. You can't use the free sites but there are hundreds of DOC sites in amazing remote areas paying $0-15 per person per night.
Most tourists tend to avoid them because although they want a
free camp site, they can't live without a toilet, shower, shops, a pub and the internet on their doorstep
Have a look here for ideas:
https://www.backpackermarket.nz/
TradeMe is a pain requiring an NZ address. But thankfully, you don't need to be a resident to buy a vehicle here - just an address will do.
I moved from the UK 3 yrs ago so, like Nigel, if I can help, let me know