Okihara,
Have a look at the Recreational Aviation Australia web site, but in simplified terms, RAOz is a self administrative, member owned body that is a facsimile of CASA for the relevant aircraft ---- of which there is a very good range. I would thing there is more active private flying in the sector than CASA private GA, and certainly more active pilots than active PPLs.
Indeed, this is where all the action is, as far as private flying is concerned.
This is the affordable and active end of private flying. A very similar system operates in the UK. In particular, medical standards for RAOz are are super simple, if you qualify to drive a private motor car, you qualify for the RAOz medical standard.
Having said that, as you intend to go to a PPL, you should do a CASA medical, and start of with an initial for a first class standard, only downgrade to a Class 2 if you are forced so to do.
There are RAOz flying schools, and many schools that offer both CASA and RAOz streams, and at a later date, converting an RAOz pilot certificate (note: not a "License") into a CASA PPL (an ICAO license) is simple and straightforward.
Rounsounds' advice should be heeded, to the uninitiated buying an aircraft can be a very seductive thought, don't get suckered in at this early stage.
Remember, to make a small fortune in aviation, start with a large fortune.
Tootle pip!!