Hey Undagekiter,
We did trip around Australia from Cooktown clockwise to Geraldton. Took us the best part of a year. Pretty fun. We did, saw and experienced just about everything including blown tyres, cooked engine, worn out clutch, electrical problems etc etc. We also had a very memorable trip and met heaps of cool people and shared some awesome experiences. We covered more than 35,000km. I'm sure there are still a few people out there that remember when we breezed through their town. And thanks much to the people who put us up and allowed us to camp on their driveways etc.
Here is the bus we did it in =>
Here's a few ideas that have sprung to mind and may help you out.
My advice is to budget for $25 per night for accomodation and about the same for food per day, then add fuel. This should be enough to get you along on a minimal budget and still have a dollar or two left over if you are frugal. Sometimes you'll camp for free and this will become beer money. Expect the unexpected and have back up money. You wont feel too independent and grown up if you have to ring your folks and ask them to send money to help you out of the sh1t. The way to grow up and find yourself is to handle the hard times yourself.

The rangers are pretty serious when waking you up at 5am in the morning and yes they WILL find you, no matter how far off the beaten track you go. They usually found us

. There is a rule that you can only camp on the side of the road where you are more than 25 kilometres from a Caravan Park, otherwise you are expected to stay in the Carapark. The fees for a basic unpowered site these days are around $18-$22 per night, $25-$30 for a powered site depending upon the park and where it is.
Most Caravan parks will allow you to have a shower and use their ammenities for $3-$4 which is nice if you've been stinky for a couple of days. If you fein forgetting your key or password at the ammenities block, sometimes someone else will let you in. You are best in a big park as they can't remember all of their campers like in the smaller parks. Hot showers are gold!
I'd look at a Mitsubishi express, they run on the smell of an oily rag and are very reliable, (you just can't kill them) and are heaps cheaper to repair than a Toyota. If you service the van before you leave, you wont need to service it again until long after you get home. Take with you a tow cable, jumper leads and a 10 litre fuel jerry just for emergencies, make sure your spare tyre is good to go and has tread. I would also strongly recommend buying an electronic laptop safe to bolt to the floor of your van so you can put your valuables in it when you go kiting. They are about $129 from Bunnings. You can keep your wallet, keys, sunnies, Ipod, mobile and laptop safe and sound. Put a spare key in a magnetic case up underneath the van (well hidden) and keep one on a string around your neck when kiting. All other keys in the safe.
If you can afford it, grab a little Evakool ice chest type cooler or one of those small Waeco fridge coolers, they keep ice for 5 days. Nothing will beat a cold beer after a big days kiting and you can keep your milk and some fruit cool and fresh. Grab a cheap 2 burner stove and a gas bottle, one saucepan, maybe a billy for coffee or tea and a fry pan and you're good to go. Have a look in the Trading Post or Quokka in WA.
You should also make yourself a grill/cage piece that you can wind up into the window frames of the front doors to lock them in place. Sleeping in a van in summer gets damn hot and you'll need airflow. Make sure it has insect screens too otherwise that one mozzie will drive you mad in your van when trying to get to sleep.

You could also put some Spinning air ventilators in the roof to help extract CO2 whilst you sleep, it will allow some of the heat out too, especially if you have company (if you know what I mean)

.
I could write a book on what to do and not to do, but I won't bore you, so last piece of advice. Things will probably go smoothly for you, However if you encounter some challenges, no matter how bad things may get, keep in mind the most memorable parts of your journey when it's all over will be where you overcame adversity, in what you thought was the moment you were going to pack it all in. Good times remain in memory too, but not as specific memories, usually just the sense of freedom and wellbeing.
Good luck on your travels and have fun.
Good winds,