Sarjapur Road pe under-construction lena sahi hai?

Under-construction projects always make me so nervous with all the delays. I'm checking out a 2BHK on Sarjapur Road, maybe a Prestige project, but ab samajh nahi aa raha ki kitna extra budget rakhna padega. Property price ke alawa stamp duty, registration, interiors, yeh sab mila ke kitna lag jata hai? Log bolte hain 15-20% extra, is that true? Aur under-construction mein possession delay ka kitna risk hota hai, especially with big builders? Koi real experience share karega?