Work out how you want to be living when you are 65 and where the money is coming from to support your income. To be financially free, that income will either come from;
Rent from rental properties
Business income
Interest from a couple of mil in cash in the bank
Dividends from Shares
Buy and sell shares
If you want to save tax, buy some shares, ensure you lose the money and claim the loss as a tax deduction. Accelerate the loss with a margin loan. (All tongue in cheek)
It's true many people do make shares work and if that is what you love to do, dabble but do it slowly so you make money. If you are going to play with shares, make sure you are really good at share trading with a small amount of your own money before you get any margin loans. Borrowed money makes the losses worse and the wins better. Same for houses.
So heading down the investment in property path (cause it sounds like you don't actually want to make this your PPOR, you can make the money to get your goal in either of these strategies - just pick one or 2 and stick with it.
1. Buy one property and pay it off...then buy another and pay it down etc (Anita Bell)
2. Buy Positive Cashflow (Dymphna Boholt and Margaret Lomas) .. save up and then buy another etc
3. Buy Negative Cashflow - You would want to buy in areas where the growth is almost a dead cert in your eyes (and around 10% pa) otherwise you could be dishing out the shortfall for years. (Not convinced the sunshine coast is set for this. I would be thinking more inner capital city) Then you pull out the equity to buy the next property.
4. Buy and renovate or develop - This makes money faster to get more properties and the cash on property sold or the equity withdrawn buys the next property deal if it's done right. And there are great developers on this forum who are doing it right.
For this house
Take your mum out of the picture. She could leave at any time and then you need to get a new tenant in. If she wanted to, is it possible that she could buy the place for herself and then she has her own place to live?
So is the house a good rental place.
Does it need work? Would a reno improve the value of the place and therefore the rent?
Can you get it at a price that will make it CF+ (important if there is not much growth in the future in it).
How much positive or negative cashflow are you?
Have you run the sums through an excel spreadsheet or calculator to work out how much money will be leaving your pocket.
How much do you think the place is worth? Have you looked at the sales of similiar duplexes in the area?
How much are you prepared to pay for it?
Is he likely to sell it to you with good equity in it or does he want every cent he can get from it?
If the property does not stand on it's 2 feet now as an investment to fit your property strategy or as a PPOR you want to live in for several years, then move on to another property.
