Hey there, you're thinking 25 foot which is good from a perspective of it not being super-critical to get everything right first go but expect this criteria to change as you get some experience.
Suggest getting onto as many boats as you can on whatever basis you can but you shouldn't have to pay someone like-minded to take you for a sail and give you some advice. Go and look at some boats, and
go for some sails on them. If there's one piece of advice I would give it's don't feel bad about kicking tires. Boats are really really slow to move and if you find a decent human being who's selling a boat in genuinely good condition, he/she might be very happy to show you the ropes a few times as part of the sale.
This crowd are offering the day skipper practical for less than 2.5K Australian in February:
pacificsailingschool.com.au/rya-day-skipper I did a similar course years ago (actually what was called RYA Keelboats 2) which taught me a bunch of essentials. I just googled that one - there will be heaps in Sydney but you should be going in with the mindset that the course will save you many times the 2-3 grand it costs to do. Saying this, your original idea is solid - especially if you back up the practical with some theory.
When I got my boat last year I went to the local sailing club and introduced myself to the tower (race organisers). Someone there said "are you looking for crew" and I have ended up with one of my main crew members being a retired former boat-owner with over 40 years experienced and thousands of offshore miles. I learn heaps from him every time we go out. Being retired he has even been known to pop down to the boat whilst I'm at work and fix stuff :)
My point being that there are lots of very experienced sailors who don't own a boat any more for whatever reason who will probably love to go for a sail on your boat with you, and be very very happy for you to be picking up the tab for insurance, maintenance etc.
If you're ever in Melbourne PM me for a sail.
Cheers,
Jon