A good architect can advise you possibility of future development.
15 meter frontage may/may not be good enough for duplex (check relevant dcp)
For battle Axe..you will neeed 3.5 to 4 meter access way.so check side set back of existing dwelling.
Buying a profitable deal in Sydney would be a challenge!
No need to pay money.. If you need help, post genuine numbers and deal on forum and plenty SS members will give you their honest feedback and guidance.