First home hunt: The hidden costs are scaring me

₹1.5 Cr for a 3BHK from Experion Developers near South City I is my budget, but I'm honestly lost on the total outflow. My parents keep saying there are hidden costs beyond the property price. What extra budget should I keep for stamp duty, registration, interiors? Is 15-20% enough? It's an under-construction project. How do you evaluate delivery risk? Should I worry about delays/quality? Any advice on builder-buyer agreement red flags for first-timers?