You'll have a ball regardless, but for my 2cents:
Wouldn't bother buying a camper van if you're only going away for six months, you could end up spending your last month sat in a car auction every day trying to hock it!
Re: where to go first...depends if you wanna bring a lot of stuff home or not - I bought a good bit of stuff in Asia that I couldn't have carried on my back for the rest of the trip...you're less likely to buy stuff in Oz as its that bit more expensive, and I'd imagine you'll be on a budget of some sort. So Oz first perhaps...
If you're getting that far over, I really wouldn't miss the opportunity to spend at least a month in NZ (and possibly a couple of weeks in Figi/Bali etc..) You can get a seven stop ticket for only a couple of hundred quid more than a two stop, but if you decide to book them while there (i.e you meet people, want to travel etc..) you'll pay that for each subsequent flight...
The Sydney-->Cairns route can be a bit misleading...a lot of people stop off in each stop along the way and while everyone has a different experience, the only REALLY worthwhile stops, if you're on a tight schedule are Byron, WhitSuns, Fraser, Noosa, Brisbane and perhaps as extras/side trips; Barrington Tops and Magnetic (although, obviously, not in that order!)....
If you do a seven stop trip, it might also be worth using one of your stops to go to either Perth (I really regret not doing it) or to Alice Springs and perhaps travel overland up to Darwin and use your Darwin outbound to catch a connector home or something...
As I said, my two cents...!
Needless to say, you will have the time of your life!
Just realised I forgot to mention Thailand...as I said, going on your way home would probably work better, but everyones different! I didn't like Samui (the sleazy aspect you mentioned) and for that reason, skipped Phuket and Pattaya etc... but PhanAng was lovely and I believe Koh Tao is great too...
Definitely make a concious effort to go up North - Thailands really not all about the islands...Chang Mai is a great city, although will take a bit of getting used to - but from there head to Pai and without a shadow of a doubt, do a trek - three days preferably...they're fantastic experiences!
Some people hate Bangkok, I loved it...each to their own.
The main thing is, no matter where you go, in the right company, you'll have a great time (and for safety etc...obviously, its a plus), and remember, Thailand is a gateway to a lot of other Asian countries that aren't as touristy yet (Cambodia, Laos, Vietnam etc...) - I have to go back for them as I fell into the Thai trap!
Have fun!