Hi Kozzie,
4wd- although nice to have- is not an absolute necessity.
As long as you use some common sense regarding time of year- and check road conditions before travelling you can get practically anywhere.
Before going anywhere remote- call in at the police station or council office- they can give you a pretty up to date road report.
Get yourself a decent CB (with a good antennae) and ask oncoming truckies what the road ahead is like- we dont mind telling travellers about
dodgy roads, stray livestock, etc.
Something like a LWB toyota van would be ideal, with a roof basket to carry masts, sails, toys, (and if you see the light...landyachts)
Toyota's have the reputation for reliability and parts being available, but mitsubishi vans tend to be cheaper to buy initially.
"Wicked Campers" sell excess campers regularly for around 4 or 5 grand- not sure about condition- but after a lot of bad publicity a
few years ago, they really lifted thier game with maintenance of thier fleet.
As others have said- Gumtree have always got campervans listed by travellers and backpackers selling up before heading back overseas.
Prices seem pretty good.- most decked out and ready to go.
I would also recommend a trailer, 6 x 4, or 7 x 4 box,
if possible- have the trailer fitted with the same wheels and tyres that suit your vehicle- so spares are interchangable.
A trailer will give room for extra stuff like spare wheels, jerrycans, and water, with stuff stored in sturdy plastic crates (milk crates?) and cover the
whole trailer with a tarp so its contents are out of sight.
Keep us posted on your progress!
Stephen.