If you are in Cockatoo Victoria then hard water shouldn't be a problem, so stainless steel tank is definitely the way to go, I have a 'Bolt on' in Lancefield, with a glass lined tank, due to hard water.
Have a at
http://earthworkercooperative.com.au, for where I purchased from.
Heat pump only required to run a couple of hours a day, so very economical as well.
The web site says it best, but the co-op is ensuring that tank manufacture is staying here, and plans are under way to manufacture some parts of the heat pump here, as well as assemble it here.
So, a quality HWS with jobs for Ausies as well.