what will sell better? 3 larger bedroom or 4 smaller bedrooms? It is near Parramatta so it's a pretty suburban area with lots of families.
4 bedrooms, especially if they are in short supply in that area. But even if it's a smaller family an extra room is always handy.

Also 2 living areas or one living area and study with a large entrance?
2 living areas - a large entrance is a waste of space IMO, especially if you're doing 4 brms, a family with a few kids wants more than one living area

Laundry with an external door or just go through the living room to the garden with the laundry?
Definitely a separate door for the laundry, having to go through the living area would bug me to tears.

Kitchen island or breakfast bar?
either, but if space is limited, a breakfast bar would be better.

Downstairs floor tiles or wood?
Agree with TMS, from an investment perspective I would go with tiles.

Are there any special features that make you want to buy a home?
We're renting & not buying any time soon, but my 2c... I would be looking at the layout & maximising space. An easily maintained garden, with a covered outdoor area. Plenty of electrical outlets. Does it have an ensuite? Because that is a deal breaker for me, we need at least two toilets.