There are some good pieces of advice here. One thing to consider if you are planning a Darwin to Kunners trip is to factor in the wet season where the road gets blocked off due high water.
Ideally you would have a four wheeldrive that you could sleep in, cross creeks, bounce kangaroos, pack up quickly. The UHF is a good idea to. The number of times i wished i had a 4wd and/or a bull bar. In a sirion you could come second best if you hit something.
I STRONGLY reccomend that you do not drive at dawn dusk or night. (Particularly in a sirion).
Whilst its a great adventure in your shoes (and having well and truly been there and done that) i'd probably fly up and buy a car.