If you come to Perth, prepare some time to tour the surrounding regions. The city itself has very few attractions apart from the beaches. The south-west of WA has many natural attractions which are good for a few day road trip.

Best places to compare flights across the airlines:
No one-stop website or travel agency I know of that will cover ALL airlines but www.studentflights.com.au is pretty good but keep tabs on Virgin Blue, Tiger Airways, Air Asia and Jetstar.
Best places to compare hotels:
Just type in 'name of city' and hotels in google. Most of the australian hotel finder websites are pretty good and equivalent in services

Or skip the extra and focus on Sydney as the one AU stopover.
Might not be a bad idea