In Australia you want to go to
Maelstrom. As for the battleforce it's both a good start and decent expansion. The armount of bits in it is pretty good as well. I managed to make a fair few conversions with it.
For Warwalkers though you want maybe 2-3 in each squadron. I don't use them because they're a fair bit slower than the rest of the force I have and don't really fit in too much with the small group that's for defence either. They're pretty good if you can figure out how to use them right. Outflanking is generally the best way to go, but don't overestimate them as they do die pretty easy.