Hidden charges builders only reveal at possession

Been renting for years and this landlord raised rent again, so finally taking the plunge. Looking at flats in Sadashivanagar, maybe Keya Homes, but prices are high. Bangalore mein negotiation kitna work karta hai? Log bolte hain 5-10% tak ho jaata hai, but is it true? Also, all-in cost mein kya-kya include hota hai? Stamp duty, registration ke alawa aur kya hidden charges hote hain jo builder baad mein batate hain? EMI vs rent ka math toh samajh gaya, but yeh extra costs dar lagate hain. Any hacks for first-time buyers?