As of now, TUV 3OO is offered with a BS4 engine. However, Mahindra is planning to introduce the BS6 engines soon enough. Mahindra XUV300 is all set to get and BS6 engine and others are lined up as well.
For the availability of spare parts, we would suggest you walk into the nearest service centres. You can click on the following link to see the details of the nearest service centres and selecting your city accordingly - Service centre. Moreover, for an idea of the prices of the spare parts, you may follow the below link - Spare Parts.