You are very unlikely to be able to use a visitor visa in your circumstances. What you want to do is emigrate to Australia, and this will probably become quite obvious to consular or border officers as soon as they look at your situation.
The tourist visa description explicitly states:
- intend to come to Australia to visit only
Be a genuine visitor
You must only intend to:
- stay temporarily in Australia
(Emphasis mine in both)
Once they notice you don’t have any ties in the UK (no work, no property), ties in Australia (your son and his girlfriend), that they themselves intend to emigrate, it would be quite normal for them to reject your visa outright.
You should probably consult an Australian immigration lawyer to determine the best route for you, but unless you intend to go back and forth between the UK and Australia and spend less than half of your time in Australia, I doubt a tourist visa is the right path for you.
Also remember that living in Australia is not quite cheap (though it varies a lot with the location). Living off the interest on the capital from the sale of your property in the UK, while still maintaining the value of that capital in the current relatively high inflation period, may be a lot more difficult than you think.