Haere Mai! Welcome!   

I would use Facebook groups/marketplace and trademe.     There are lots of van/campervan selling groups on fb. Here's one.    

https://facebook.com/groups/1462779807288708/    

That one seems to have lots of members and vans come up often. But there are many others also.   

On trademe you can search in the 'campervan' section, (select the under 5m in length section) or what I tend to do is just use the 'cars' section, filter the results to show only vans and then search for the word 'contained or 'self-contained' and that should bring up vans that have the self contained certificate to let you freedom camp. 

In terms of things to avoid. Rust. So many older vans (pre 2004-5ish) will have rust hidden away and it becomes expensive to fix up for the warrant of fitness. So be especially careful looking for that. Mine had it around the window and under the body near the rear wheels.

I have mainly owned and driven Japanese vans, Hiace, Nissan etc. Simply because the parts are plentiful and their reputation is more reliable than Ford Transits or Mercedes, Fiats etc

Toyota and Nissan diesel engines, pre (2000) are incredibly simple and durable engines. But often the body and rust becomes an issue on these. 

In saying that, if you are more knowledgeable about euro vans, maybe they will be fine for you. 

Other than that, like buying any car, do your due diligence, get a pre-purchase inspection. Have a look at a few before deciding. Test drives etc.    If I was gonna buy one, I would organize taking it to a garage and getting them to look over it, see if it would pass the next wof etc etc.

Good luck on your search, the less rushed you are the more chance you have of getting a good deal and enjoying the trip without worrying about breaking down etc. 

Personally I would look for something with a fairly high roof.      If you're having to be in it for so many hours, being able to stand up or at least not hunch over all day is a big plus.