What do I need to do when I buy a car in Perth WA?

I am traveling to Australia (Perth, WA) later this year. And am planning to buy a camper-van there.
What do I all need to take off in this process? Insurance, registration etc.. and where do I do all this?
Is there a preferred insurer to use? Is there a preferred supplier for road-side-assistance?
I am driving from Perth, through the Nullabor, to Adelaide, Melbourne, Sydney and Cairns.

And am planning on selling the car there again.