You could pick up a pretty good deal at this time of year, as shops are trying to clear 2012 model stock to make way for the 2013 range which is beginning to filter into the market. Most WSD MTBs above base level have similar design variations from the mens version as road bikes; shorter top tube, narrower handlebar (most MTBs are trending towards loooong handlebars, to a point I find a little excessive) and wider women's saddle. The top tube won't be the traditional ladies' low step through, although some manufacturers "dip" it slightly to give a bit more standover clearance.
The Giant Talon series* is a good mid-range set, which includes two women's models. The aluminium frame, while no lightweight isn't absurdly heavy either. Fork travel is 100mm, which is standard for a Cross country (XC) type MTB, and can be locked out (to make it a rigid fork) via a switch on the fork crown. They both feature disc brakes; those on the Talon 1/W are hydraulic, which are a lot more effective, both in outright stopping power and controllability of that power (modulation). The RRP of $899 is a little bit outside your range, but you should be able to get one a fair bit cheaper. The Talon 2/W normally sells at $649.http://www.giant-bicycles.com/en-au/bik ... 451/52902/http://www.giant-bicycles.com/en-au/bik ... 451/52899/
*The shop I work in sells Giant bikes.
EDIT: Re Cyclocross (CX) - It depends how & where you plan to use the bike. If you are going to be on smoother stuff such as Yarra Trails or the Kurrajong section of You Yangs, then yes a CX will actually work quite well. But if you do plan to explore more technical stuff, the suspension & fatter tyres of the MTB will be a better way to go.