Gurugram flat search: I had no idea about these costs

Since January I've been shortlisting flats in Gurugram, and honestly, the process is overwhelming. I'm looking at Sector 46, maybe an M3M project. My big question is about hidden costs. Beyond stamp duty and registration, how much more should I really budget for interiors, PLC, and all that? It feels like it just keeps adding up. Also, what red flags should I absolutely watch out for in the builder-buyer agreement? Any advice on M3M or general tips for a first-timer?