Hi
I would suggest going with at least a 6'' Dob or an 8'' if you can pay the extra.Have you herd of the Saxon range of scopes there are fairly good quality for the money.Have used these scopes before and are sturdy and with good optics.You mentioned you are from Brisbane.Our Society holds regular viewing nights on the northside and southside each month called Urban Observers which have been setup for helping Members of the public to learn the night sky and with help of equipment purchases.
You do not have to be a member of our Society to attend and are free of charge.
Info can be found at
www.seqas.org
Regards
Dave