In reply to Lynsety:
To be honest, if you're only going for seven weeks, then buying isn't really a good idea. Selling them on isn't really that easy, we met lots of people for who it took well over a week to sell their's and that's significant time out of your holiday.
Also, if you're going now for a couple of months, you'll be buying close to the start of high season but selling closer to the start of the low - the wrong way around!
If you do decide to buy, then I'd avoid the undercover market at all costs, you're reasonably likely to get ripped off in there. There's a better market (Tuesday mornings?) down on the front.
Renting is probably the best way to go though. Juicy rentals were who we went with (after we scrapped our shagged out van!) and they were good. If you're going in the southern winter, spend the extra $10 or so and hire the snow chains from the rental company as they're expensive to buy and mandatory on some roads in winter (ski fields, Milford Road etc).
I'd avoid Wicked at all costs. They don't seem to work out any cheaper, have appalling customer service (by most accounts) and you're unable to choose which crappy picture you have on the side of your van and some are quite offensive. Also Wicked Vans scream "I'm a tourist please rob me".
Good luck!