sumatra - NZ is geared up big time for campervans, and is perfectly suited for them. There are about 5 small bits of road in the entire country which aren't suited to them and the rental companies give you maps with them clearly marked - they don't want their campervans damaged any more than you want to damage them.
We did it for a few weeks and - as you hint at - we found the freedom to stop off at some seafront or mountain view and have our food and a break or go for a walk to be great. So much so that we bought ourselves a campervan after coming home.
On the South Island one of the big scenery things is Milford Sound - it is a big destination for tour buses and groups. It is pretty impressive, but we reckoned that a lot of the scenery we passed through coming up from the south was just as impressive.
We did try a lot of NZ wine, but to be honest nothing stood out - it was a few years ago.
In terms of campgrounds, you will find at least one in most reasonable sized towns. There is a chain called Top 10 - [broken link removed] - which has a place in most of the usefull towns, and we stayed in a good few of them. We eventually decided that because they were part of a chain they were actually too generic and too full of tourists (which we of course were too) and not enough NZ'ers. There is another more independent chain which you can find at
http://www.hapnz.co.nz which had camps which were less fussy but more earthy. Depends on what you are looking for. By the way if you do go either route, they both have discount cards which entitle you to 10% off which pays for itself after 2 or 3 nights so it is worth getting. It is also dead cheap (or cheap as chips as they might put it) to camp there.
Cheers,
z