Hi Sally,
Renting in Rossmoyne is approx. between $800 and $1000, although the rentals are reducing at the moment... Best to contact the Headmaster of the primary school, Geoff Anderson, and let him know of your plans. That's what we did for our 3 kids and he kindly booked their places for us when we arrived. For the eldest child, so long as you are in the catchment he will get into the High School.
In terms of private schools in the same area, All Saints has a great reputation as does Kennedy Baptist College, the former being approx. $14k per child and Kennedy is $6k.
Hope that helps.