Hallo Sara,
I wouldn't bother buying a van for 3 months. Long term rental shouldn't cost more than $30.00 per day. You might get even better rates in Deutschland. Here are 2 links:
www.hippiecamper.com/www.wickedcampers.com.au/Try the others too eg. Apollo, Britz etc.
We still have wind here in Port Douglas and the Cairns Beaches in October. It's getting quite here at the end of November and that's a good time for wind in WA.
Man sieht sich...
Tsch??
Caesar