Documents to check before buying an under construction property
a) Land records, b) Land use type, c) construction clearances, d) layout plan approvals, e) master plan of the area, f) no objection certificates and other clearance, and g) booking form, allotment letter, and builder buyer agreement.
About registration you can get the information here: https://www.commonfloor.com/forum/501b6933e7a95-regarding-registration-cost---vat---service-tax