Rights of legal heirs of sale-cum-gpa holder sir/madam,

Sir/madam, i am Suresh from Tirupati. my father bought a piece of land (3 residential Plots) in the year 2010 from a person. my father paid all the sale consideration amount to the seller and instead of getting sale deed, he got a registered agreement of sale-cum-gpa from the seller, just to save registration expenses, as my father wanted to sell off the plots subsequently .later in the year 2012, my father expired without selling the plots. now we are four, one mother and three brothers left behind as legal heirs .now when are trying to sell the plots to others, some are saying as legal heirs, we can sell the plots and execute sale deed in favour of the intended buyer/s. some are saying the gpa was automatically cancelled as the agent (agreement of sale and gpa holder) died and original owner, principal, will have the right to sell the plots. our doubt is, as my father paid the entire sale consideration, had not he had the rights to sell the property, even though it was clearly mentioned in the deed itself, that the entire sale consideration was received by the principal. it was also mentioned in the deed that the physical vacant possession was given to the gpa holder and he can sell and register the property to someone who is interested or execute deeds in his own/or nominees favour. we want to file a petition for a legal heir certificate from the court. after getting the legal heirs certificate from the court, can we, the legal heirs can execute the deed in other's favour or get the registered sale deed in our favour and then sell the property to others. we tried hard to find the original owner, principal. but his whereabouts are not known. what shall be the course of action, if the principal is not available. can we make a petition to the court to declare us, the legal heirs, as the owners of the property/title. in such case, can we appeal simultaneously for legal heirs certificate and declaration of title, to the court. if we need to make a petition separately for legal heirs certificate and declaration of title, what will be approximate time and money needed. please advise.