Well i think you have come to the right place!!!!!
There are the two observatories (in know of) close to Perth:
1)
www.perthobservatory.wa.gov.au
2)
www.ginginobservatory.com (this one north of perth a bit, But a lovely drive)
I would look at to stay somewhere near the Swan Valley. Lovely part of Perth and many a winery to choose from for some beautiful wines cheeses. Lots of bed and breakfasts to choose from. Also the Valley is about 20 mins from the city, so darker skies. (and really perth isnt as bright as the rest of the capitals around Australia anyway, so even better)
The website I use for the weather is
www.bom.gov.au. They are normally pretty good.
Happy Viewing
Benno