You won't find too many good, cheap set of wheels in Darwin. You WILL find a lot of very high km vans and old Falcon wagons, etc, that have belonged to backpackers, and that they now want to quit, because they've finished with them. Sometimes you can get a bargain if you catch them as they are getting ready to leave. Remember that waving a few notes in cash works wonders. If you do a deal, and need to go get more $$'s or a bank cheque, remember to seal the deal with $50 or $100 deposit - or it might be gone by the time you get back.
Gumtree is pretty good for sussing out stuff - there's a 1994 Camry there with a few dents for $400. You can't kill that model Camry, and if you ain't fussed about appearances, it will get you around, until you find something better.
Used Cars in Darwin - Gumtree Free Classifieds - 1