It depends on what type of helmet you have.
The UK4AA (LED or Xenon) torch tends to be the standard, and works very well for structural applications - it retails for around $50. You can also buy smaller, cheaper LED torches, for about $20-$30, but they tend to perform worse in smokey/foggy conditions.
As for how to mount the torch, there are many different options, and it will depend on what helmet you have and what incidents you tend to attend. If its just rural jobs, and a rural lid, then mounting the torch with a fixed mount eg: The Blackjack will suffice. If you are doing structural/rescue work where you want the torch to be movable and generally point in your line of vision, then you should probably go with one of the many movable mounts. Further to that, some mounts don't work so well on certain hemlets, and may need a little bit of in-house modification work.
You best bet would be (if possible) to head to somewhere like Fire and Rescue Australia, or to a safety equipment store, with your helmet and talk to them about what will work for you.
Here are the torches and mounts:
http://www.firetrader.com.au/category44_1.htm