For this, we would suggest you walk into the nearest dealership as they will be better people to assist you. You can click on the following link to see the details of the nearest dealership and selecting your city accordingly - Dealers.
The Duster RXE and RXS differ from each other on the basis of features offered in the car, otherwise, both variants have the same engine displacement, mileage, and power. On the other hand, the RXE 85 PS os offered with 85PS of power with more mileage than the other two variants. For better clarification, follow the link to the detailed comparison of the variants - Compare.
Follow the below link, to get the details of the offers and discounts (If Any) on the Renault Duster - Offers and discounts. Moreover, you may visit the nearest dealership for confirmation. You can click on the following link to see the details of the nearest dealership and selecting your city accordingly - Dealers.