if it's possible to buy a flat for the same money as it does to rent in Hong Kong or are there many hidden costs I don't know about?
The main problem you would have is that banks in HK will normally not lend more than 70% of the property value. Even with additional insurance the maximum loan is 90%. If you can come up with the large down-payment required, then interest on the mortgage would be similar to the rent. Principal repayment, especially in the early years could also be a problem, and negative amortising or interest only loans are not readily available in HK.
Even if you could afford the down-payment I would not recommend buying in HK, one of the most expensive property markets in the world.