If you haven't already done so........I'd strongly recommend getting in touch with the Archicentre and get one of their reps to have a bit of a squiz. Bear in mind that they are mostly just architects as opposed to builders but there'd be little in Sydney they haven't already seen.
Hate to tell you but in Sydney this sounds like a pull down proposition.....restumping/reblocking a house is rather expensive ($100 a stump?), takes months and leaves you with a level house but one that often needs new flooring (because of broken tounges and gooves in floors), new walling (because the internal cladding has crackeed up....and new ceilings for the same reason.
People don't ever seem to realise that doing what's right is no guarantee against misfortune