document verification & untrusted landowner

Hi, I am purchasing a resale flat (I am 2nd buyer) from a Builder land owner share. 1st buyer has booked the under-construction flat. I want to know whether any outstanding from me is their or not. The first buyer has made assignment agreement, landowner KYC, MOU and NOC. And now flat is almost ready to move(95% complete). Now I want to verify those documents. Builder has told me to have trust b/w you & landowner. Registrar office does not have land owner details. And land owner is not sharing property file & flat cost break ups or JDA; so whether GST is paid or not, whether GST has to be paid 1 time or 2 times, whether Car parking is half paid or full, whether any due before registration etc etc. Please advise.