How do I calculate my buying power before choosing a property?

Use a simple framework: cash available for the transaction plus the reserve you are willing to deploy, minus fixed transaction costs, gives you the amount available for the down payment. 
Then define a monthly installment you could still handle if your work, rental assumptions, or market timing change. 
Finally, connect that number to your purpose: end use or investment. 
A disciplined buyer sets the safe price band first and only then chooses the property type, area, and project within that band.
This single step prevents a large share of buying mistakes.