Seller is deceased and require consultation for that

I am about to finalise a property but seeks one major problem i.e. Property is in the name of single applicant and that seller is deceased and before his death he has also entered into a registered WILL authorising his son to take all the responsibility of certain properties including this one and other legal heirs of deceased seller are his three daughters and a wife. Three daughters and a wife are not entitled to handle this particular property as also mentioned in registered WILL. Whereas they have also submitted some documents to builder as per check list to change the applicant name in the property as it is a under construction property which will be transferred by builder only. Builder will take some days to change the applicant name. So my question to the experts is 1. Whether I should give the son some advance amount or should wait till the applicant name changes in the builder record? 2. Shall I have to get some documents signed from seller all legal heirs like. relinquishment deed or something to ensure my safety in future? 3. Can any other legal heir like any of the other three daughter or wife can be a problem in future and can stand of their right? can possibly can be done