Your planning and research has been super impressive - kudos to you
As for telecommunication providers, the main players in Australia are Telstra, Optus and Vodaphone - if you are planning to work in rural areas, you will want the most network coverage so Telstra is the best bet. But Telstra is more expensive than the others. For data only SIMs, I'm currently using Vaya which runs off the Optus network. Another similar player is Amaysim - reasonably cost effective and runs off the Optus network as well.
I am from Melbourne so I cannot comment too much on the cost of living in Sydney - while on holiday there, it is definitely higher than the other capital cities depending on where you end up staying.