I'd be going for Bonds suits for night time (not sure about Sydney but in Brisbane it's still quite warm during the day until late March - early April).
And all-in one suits for the daytime - get a mix of short sleeve/short leg and long sleeve/short leg.
I'd just buy what you can get away with in 0000 and 000 because you'll probably need warmer stuff for winter. By then bub will probably be in 000 mainly.

I think a good rule though is to buy about half of what you think you'll need. I bought way too much - both times - and I've got baby clothes stashed away that have never been worn!