1. You shall have to understand that risk and quantum of risk in buying the property without the legal heir certificate,
2. If you fail to collect it then after you purchase the property from A, anybody may appear to claim that he/she is the step brother of B and is claimant of share of his father's property. This is a hypothetical situation,
3. If there is a legal heir certificate, that risk is eliminated,
4.It is not very difficult to collect the legal heirs certificate wherein the legal heirs of B's father will be mentioned even now,
5. The decision to buy shall have to be taken by you depending on your risk bearing capacity and the ground reality.