Hi Jason,
My girlfriend and I travelled Australia mostly by car ;last year.
We found Europcar to be the msot cost effective and "reasonable" to dealwith.
I say reasonable because you "will" find from time to time that you are a day or two early/late to your next destination....and some of the penalties are severe, so check that bit out in detail.
Europcar will allow you to drop the vehicle off in Cairns if you want, not many will.
As for comfort, the medium range are the best, but more expensive.
"But hey your in Oz maaaan, who cares for comfort when youre living the dream"......thats okay for a bit, so get the comfort you want, its much better on the overall mood you will be in after sepending most of the day in the car.
Lastly, I would definitely book your first two places in advance and then book by email as you will visit and hear about other places you did'nt think you wanted to stay in, but will hate to leave or must visit.
I am getting great flashbacks writing this, so thank you for that.
Best of luck.
Slimbo