Hi, RP is an absolute must.
For new car - U require Passport copy, bank statement for 3 months, Salary Certificate, No Objection letter from your company, Quotation for new car.
For Used car - same as above if you are going through Bank for a loan. Additionally you will require Quotation from Used car showroom. Clearance letter form the Sellers bank.
If you are paying cash, you dont need Salary Certificate or Bank Statement or Quotation. But you will need your RP details and Vehicle Registration card from seller.