What is the grant letter concerned with this property, was this property granted or assigned to this owner by the government?
Did you obtain a legal opinion to confirm the genuineness of the property on the basis of the papers produced before you?
If not then you may first obtain a legal opinion and clarify the importance of the missing grant letter because the advocate giving his opinion after scrutinising all the documents would be the better person to render a proper opinion and recommendation to buy this property.