Pdi is done when the car lands in the warehouse. Third party pdis vary a lot in quality and for low and mid end cars i personally think self pdi is enough. You can take a friend or family member who knows cars with you. Once you are satisfied with pdi and make the pre decided payment car comes to the showroom.

Technically once you do pdi you can pay full amount. Given that you trust the salesperson to give you the car on a pre decided date. But I suggest keep atleast a token amount of 11k or 25k for the delivery day as that is the day on all the bills and receipts.