[Cliff]

We are not sure whether we should hire a campervan?
Hire is maybe you best option. Especially true if you are coming in the next few months (Winter) and the prices are cheaper.

Buy a cheap second hand campervan (we do intend to do lots of camping when we can!)? Please also give suggestions of places to see, routes to follow, campsites to stay over, places to eat etc.
Just follow state highway 1 from Auckland to Wellington and you will see a lot of the country. Petrol is now $@NZD a litre.
There Is it safe to just stop off on the side of the road (if we are not breaking any traffic rules) and sleep there for the night?
No It is ok.
How good are the roads?
The main roans are good and you will not have any problems.
Any special rules when it comes to hiring or buying a campervan? Just shop around there are many suppliers.

One issue is crossing the ferry from Wellington to Picton to get to the south island. Some of teh major suppliers will allow you o leave the unit at one end and pick another up on the other end saving the cost of the ferry for a vehicle. Once out of Auckland the drivers on the will be a lot better and more courteous.

Sometimes you may also find a better deal from South to North instead of North to South so you might ask about that option.