There is no substitute for experience, Pat. Get down to your local yacht club / marina, and see if you can get yourself on board a yacht as a "begginer" crew. Actually, most yacht clubs run courses fro beginners, and that might be a good place to start. After that, most yacht clubs run midweek summer "twilight" races and you can usually get on board a yacht for those (they are less serious than most weekend pennant races, so skippers are more likely to take on "green" crew.
Don't be too fussy about the boats that you sail on...experience in a variety of sizes and styles of boats will help you to decide what is going to be right for you.
In terms of price for a second hand boat...well, anywhere between $5,000 and $5,000,000. Seriously, price depends on so many different factors, that your range is pretty much whatever you want to spend!
To get a decent idea of what you get for your dollar, there are some good websites for listing 2nd hand (and new) boats. Being in Australia, you should probably check out
1. The Yacht Hub http://yachthub.com
2. Boat Point http://boatpoint.ninemsn.com.au/portal/ali...topDefault.aspx
Frankly, if your plan is not going to get underway for 7 or 8 years, you don't need to worry too much about costs at this stage. Just get out on the water and learn to sail!