Site visit: What did you wish you'd checked?

Okay I'll be honest — I completely underestimated how confusing this process is. I'm looking at a 2BHK with KS Builders in Hindan Residential Area, and my parents keep saying it's a good choice. But when I go for site visits, I honestly don't know what to properly check beyond just the flat. What are the critical things you wish you'd looked for? Like, did anyone check water pressure, mobile network coverage inside the flat, or actual power backup capacity? Builders don't really volunteer that info. Also, how much extra did you budget beyond the property price for things like stamp duty, registration, and basic interiors? I heard it can be a significant chunk. Any practical tips for a first-timer would be amazing.