The Mahindra Scorpio N would meet your requirements of a diesel, rear-wheel-drive, 6 or 7 seater, while the low to mid-variants will come in below your Rs 20 lakh budget.
The Scorpio N has few options in a 6-seater configuration, but multiple 7-seat variants are on offer that you can choose from. Given that you drive mostly on highways, the strong performance from both the petrol and diesel engines will easily satisfy you. Also, between the two, we would recommend the diesel given its better efficiency while still delivering well on refinement and performance.