I have had my car lowered 40mm with Herrod springs. Cost me $600 including installation at a local Ford dealer. No problems with warranty seeing it was installed by Ford. Also no problems with my insurance. I am running stock 16" inch wheels so there is no rubbing problems. You will only have rubbing problems with 17" wheels if you have the wrong offset.
There are quite a few options for aftermarket suspension setups. You mentioned that you wanted the best for your car, to me that implies you will be going for a fully adjustable coilover setup. That is gonna cost you about $1500+++. But I know you are are just referring to lowering springs.
I have done a bit of research on my suspension upgrade before I ended up going with the Herrod springs due to the fact that they were available locally, and were easily installed by the Ford dealer with no fuss. As for some others, check below, but I am not sure of costs. You would have to follow that up yourself.
There are the KW springs which can be sourced from Infocus Motorsport which you are aware of. KW also make a range of coilovers aswell as struts/dampers for the Focus.
Ford Racing also have a range of springs, dampers and coilovers which are just in fact rebadged KW products.
You are also aware of the Eibach suspension setups which include a range of lowering springs, to total suspensions kits which include springs, struts/dampers and sway bars. I do not know where you can source them from in NSW.
Then there is H&R lowering springs which is available via TCCA Motorsport in NSW. I was looking at the popular H&R cup kit which will really lower the car about 2.5 to 2.75 inches. TCCA also source the very nice Rieger R-RX body kit
There are also Apex Springs which was available through Donnellans in Victoria. No idea who does them in NSW.
King Springs is another brand available through Donnellans.
You could also goto you local Pedders, as there are a range of springs for the Focus through them.
Whiteline are quite a popular aswell in the modified car scene in Australia. It might not hurt to give them a visit or call (I dont know how far you are from them).