Documents required for purchase of old flat.

We are planning to purchase 27 year old property in Pune. Current owner is the third owner and we will be the fourth owner. construction was in the year of 1987. First party and second party "Agreement" documents are there in which complete details about the two deals are mentioned.(That time sale deed was not there , so only single "agreement" document is there. Now the current owner is the third owner. He has agreement to sale between him and second owner. However sale deed is not prepared(he baught the flat in 1996 and according to him that time concept of sale deed was not there). He has a possession letter which second owner has written to the chairman of society mentioning that flat is sold to current owner and full payment is recieved. It also has a payment receipt which has payment schedule on it.(this possession letter is on stamp paper). Are this documents are complete in case we want to go ahead with this property? Remaining all documents are available with the owner like society share certificate, completion letter, commencement letter,index 2 etc. Kindly guide me.