Property and/or monetary transactions

I am a Brahmin (Hindu) and a resident of Bihar, presently I live in Delhi with my family. My father had a history of taking Alcohol. Recently he had brain hemorrhage and he underwent a brain surgery. He is fine now (do I also need a medical certificate that he is mentally fit, which he is) but my family and I suspect that it is quite possible that someone (my real/cousin uncles or anyone else for that matter) may have taken his Thumb Impression or his signature on a stamp paper or a plain paper (or any legal paperwork) to claim our property or to show that my father has done monetary transactions while selling out our property or to create problems in the due process. If that happens, how can I counter the situation and how a legal paperwork could be done to get ill deed NULL & VOID. I have already recorded (Audio-Visual) my father's statement in which he says that consciously, he has not done any transaction of any sort and if anyone shows any such document, it will have to be NULL & VOID and appropriate legal action can be taken against the claimant. I have also taken a similar statement in writing on a A4 plain sheet from my father. How much would it cost to get such a Legal Document prepared to be on a safer side? please suggest.