AMBR in Sector 135: What am I missing beyond the flat price?

Sector 135 keeps coming up every time I shortlist something. I'm looking at a 2BHK from AMBR Construction there, still under-construction. Honestly not sure how much extra to budget beyond the flat price. My parents keep saying builders hide a lot. What are the real hidden costs for stamp duty, registration, and interiors? Also, how do you evaluate delivery risk with under-construction projects? Don't want to get stuck with delays.