I myself am in Brisbane, and am about to order 19 moulds. 
Hooray for the dollar.
I have the Gothic moulds (about to order the arena and one of the accessories moulds), and they are a great choice for 'deep' dungeons and castles.
I'm going to pick up some of the Fieldstone set to play with, as they give a nice variety with very few moulds, although they're more of an outdoorsy sort of build in my eyes.
No dwarf in his right mind would build with pebbles, I say!
It depends what you want to do with them really. Take a look on the 'Post your pictures here' section and you'll get an idea of what each style is capable of.
Looking at Caveman's small brick dungeon makes me want to change my order, but perhaps next time.
One thing I'd look at is floor tiles. I originally got the cracked, worn tiles, and the smooth tiles. Having variety in floor tiles makes a lot of difference to what you feel you can build.
I've been using the cracked ones for everything because my old dungeons I've been building have no use for smooth ones. 
Thus I am investing in the chipped tiles and cavern ones this time around!
A few basic moulds  and a couple of accessories moulds gives you plenty to work with though.