Gurgaon first-time buyers: What hidden costs did you face?

Since last Diwali, I've been checking out properties in Gurgaon, especially around Manesar. Adani Realty ke projects dekh raha hoon, mostly under-construction. Honestly, I'm worried about hidden costs beyond the flat price. Stamp duty, registration, interiors, yeh sab milaake kitna extra laga? Is it usually 10-15% of the property value, ya aur zyada? Not sure if I'm budgeting enough. Also, under-construction mein delivery risk kaafi sunne ko milta hai. How do you assess if a builder like Adani will deliver on time? Kya koi specific red flags hote hain jo check karne chahiye?