Which well documents should I ask to see before buying?

Two documents matter: the notification with its reference number and the registration certificate from the basin directorate. With both in hand, the well is on the register. If one is missing, that is no reason to walk away, but it is a point for the price talks and for a clear clause in the contract. If you intend to drink the water, add a laboratory analysis; it is not part of the registration procedure but a separate precaution.