What season to get chicks? Mild winters - does it matter?

I live in Sydney, Australia, where we only very rarely get any frost. I want to keep about four pet chickens.

I want to get baby chicks so that we can get them nice and socialized (I have two little kids, so this matters) and I'm wondering, considering I won't be showing the birds, whether there is a wrong/right time of year. I'm guessing that Autumn isn't ideal, but I really want to get them as soon as possible. I will have a brooder box in my outside laundry and an un-insulated wooden coop (probably one like this, coz I'm not handy enough to make my own and I want to be able to walk in).

I'm planning on getting 4 chooks - probably 1-2 Australorps but I haven't decided on the other breed(s) yet so suggestions are welcome.

I know that sexing day-olds is nigh-on impossible - should I get 6 babies and assume that I'll have to get rid of two roos (we can't have roosters in the suburbs)? Then I can hope they are all girls and I get to keep them ;-) (chicken math seems to have hit already!).

Sorry, this is longer than planned, and I've read a bit but I'm a total chicken newbie and I have so many questions!