Sohna 2BHK: What's the real all-in cost?

Is Cosmos Infra's completion timeline for Sector 28 projects actually reliable? I'm a first-time buyer looking at a 2BHK there, budget around 70-75L. Honestly clueless about hidden costs. Beyond the property price, how much extra should I budget for stamp duty, registration, and interiors in Sohna? My parents keep saying it's way more than I think. Is 10-15% enough, or should I aim for 20%? Also, any tips for a site visit to catch things builders don't mention?