what are the documents to be verified before buying a plot

You should start from Revenue records of plot. Check the revenue records.  As per records demand the vendor, all the concerned registered Deeds.  Study the Sale Deed of vendor.  If plot is within the limitation of municipal corporation demand the property tax or other municipal  documents.  In order avoid any fraud give public notice in two or three local news papers.  Take search of property with the revenue authority.

U check and search last 30 years revenue record and check all the documents. Thereafter u search sub-registrar office record and give a public notice in news paper for this property. for example u cheack the following documents :- 7/12, no.6 entry of hakka patrak, N.A order, Plan Pass Copy, Permission for construction etc....   

You need see all the revenue recrods pertaining to the property throughly.  at lease 30 years of revenue documents needs to be cheked.

I agree with the experts. But all said and done, you better get the legal opinion from an Advocate.