I doubt one suit will do you - Qld is a very large place with a wide range of water temps! Brisbane divers tend to dive in 5mm steamers (wimps) in Summer and get into thicker suits or dry when it gets "cold" in winter when it hits 19C. I personally can handle down to about 19C in a 5mm before it becomes a problem. 19C is about the lowest in Brisbane, it gets warmer the further up the coast you go, no way you'd want to be in a 5mm off Cairns in Summer!.
Below 19C that I get into a 5/7mm semi-dry and a hooded vest - they make a big difference. I've dived this down to 15C although on on skills dive - where you don't move as much and thus generate less heat - I ended up blue and shivering at this temp.
So for Sydney you'll probably want a 5/7 semi or similar for the colder months, and a 5mm steamer for the warmer. I would wear a 5mm over the 5/7 any time I could due to freedom of movement and loss of lead.
What you should do is ask your LDS what they rent, and what they wear. Everyone is different, but that should give you a good idea.
Check out wetsuitewarehouse - they are in Brisbane, but sell online. They are generally much cheaper than your LDS. Also look to buy "out of season". So buy the thinner suit now when it's cheap and unpopular, and the thicker suit in Summer etc. When I first started diving my LDS suckered me into a $700 semi dry. Once I learnt a little, I got a 5mm, a 5/7 semi, and the hooded vest for the GF for under $500.
Another option is these guys who will make you a custom fit wetsuit (not a semi though, shame) for around $350 ->
http://rubberjungle.com/
If you have to do with one suit - get the semi dry, and a good hooded vest like a Sharskin or Probe or separate hood and vest. That way you can switch out the layers. In the worst case, you can cool down by flushing the suit, you can't warm up in a thinner suit.