Land Purchase Regarding

I am going to acquire land measuring an area of 5 acres. Access to the said land is through another land belonging to a man of SC category who is possessing an area 1 acre 22 dismil. I want to purchase the only 7 dismil from the said land to have the road to my land which I am going to purchase. This land (1acre 22dismil ) is in the name of a person, who has expired for a long. He has 4 sons and 3 daughters now. Only one son is residing in the native village and all others are staying in AP state since 1976. This man now wants to sell this 7 dismil area to me. Now the question is 1.Whether he has the legal right to sell the land by acquiring a legal heir certificate. 2.Whether legal heir certificate can be issued to him even though he has brothers and sisters. 3.How shall I purchase the land that shall be legally viable.