Welcome.
1. In short, there are three licence types common to recreational flyers:
The first is RA-Aus certificate, which is not issued by CASA. It is a cheaper way to learn to fly but there are some restrictions. You are limited to one passenger. Not recognised internationally either. RA-Aus are a self-administering body that you have to be a member of.
The second is the
CASA Part 61 RPL. It is similar to a PPL, but with some restrictions. It is also, as far as I'm aware, not recognised internationally. So if you wanted to fly in the USA they don't recognise this licence type.
The third is the
CASA Part 61 PPL. The minimum hours required is higher than an RPL, but the privileges are greater. If you fly internationally it will be recognised as a private licence equivalent.
Your goal of flying around Australia can be done on any of the above types, although you will need navigation, controlled airport, and controlled airspace endorsements unless you hold a PPL.
2. You sure can. You don't need to do the RPL theory exam or flight test if you then go on to do the PPL theory exam and flight test, although the minimum aeronautical experience to sit the PPL flight test is higher than the RPL.
3. Google is your friend. Avoid anywhere where you have to pay up front. Interview the flight instructor and if you're not happy, don't spend your money there - it's a lot, after all!If you're on the northside, I'd strongly recommend the ~1 hour drive to Central Coast Aero Club - uncrowded airspace, good fleet, experienced instructors.
4. Your age is no problem. Don't give up on your career dreams either! Whilst it doesn't get any easier with age, many people have changed careers in the late thirties, fourties, and later to fly professionally. It can be done, it's just a slog. But you also bring life experience which many very young, very green CPL grads lack...
Good luck!