How to verify or trace ownership of land to be developed by builder

I visited a property in Mumbai This is the Builder's 1st project. He is taking pre launch booking @8800 per sq ft . The builder has not got any rera registration no iod no cc which he says will be made available to us after 8 to 9 months. The said plot of land belonged to Mr. XYZ .I met with local corporator who advised me against buying flat in that project saying land is disputed belongs to central govt and salt pan land where there was hindu cemetery.f . Could you please give your opinion and how can we make sure land is not embroiled in legal complications and will project be completed in time.What legal action be taken?