What papers to look for while buying a property ?

Hi, I am looking to buy a property. It has not received OC yet. I am very much inclined to the project as it has very good amenties. Quality of construction looks nice and has 83% open area. It has 2 building of (12 floor and 13 floor). I love the serene and peaceful presence and greenery over there. Builder claims following : - Doesn't have Landowner share so he can sell for less. - BBMP clearance - EC clearance - A-khata Land - also claims OC approvals have come and some sign are pending. He expects OC in a month Question : 1. What documents shall i ask for verification and surety that property is legally ok. ? 2. Shall i wait for OC to come Or buy before it comes ? 3. He also tells me right now he is offering discount and post OC he will increase the rates. Whereas i felt if 12% GST is saved he should offer less amount after OC ? Correct me if wrong 4. How to check how much area is under loading and how much is carpet area exact ? I got to know they had initial approval of 9 floors but constructed 3 more floors. I am planning to buy on 4th floor. Sometimes i get this doubt that they will not get OC. Now how to get an understanding or some doc to verify the same. They claim they will get in a month. I have numbered the questions so that you guys can answer it easily.