There was a Whites GMT on Gumtree, under $400 last night in Victoria. Looks to have been sold or listing removed. This would have been a good buy.
If you have a detector shop near you, they should be able to assist. Sometimes they have some good second hand units available.
Depending on your budget, from what I can see a good second hand Minelab SDC is one option. Expect to pay at least $2500; however, last month one came up in a pawnbrokers at $2000 pick up only, in Vic.
You can sometimes see a the Whited TDI OZ Pro come up for a reasonable price, usually around $1500 which is about half the original cost.
As you are new, I would compare many of the popular models from Minelab, Whites, Garretts, etc. At least there are plenty of reviews for these brands on the web.
Good luck with your detector hunting.