Well, it depends on how big the house you want, generally,
1) every HKD$1,000,000 borrowed --> $8,000 repayment every month
2) for downpayment: if you are buying a flat worth HKD$3,000,000 and you are having a mortgage of the same value:
then the downpayment will be : 30% ie $900,000
3) other expenses: stamp duty and property agent's fee: another $50,000 to $100,000
If it is your first time to buy a flat, you may consider those housing estates which are convenient and easy to re-sell in the future.
Hope you'll find this useful. Good luck and hope you'll have your lovely new home soon!