My monthly running is just 500-600 km with about 50% city and 50% highway use. I am a car enthusiast looking to buy a powerful automatic car with good performance and decent mileage within a ₹20 lakh budget. Please advise only automatic cars. Is it worth buying a diesel car, considering DPF issues and the 10-year diesel cap concerns?

For DPF regeneration, it's not the number of kilometres you drive in a month that matters, but the driving pattern. To avoid DPF issues, you need to be driving at steady speeds of 50-60kph for around half an hour for the DPF to ‘light off’ and burn the soot accumulated inside. 

Since your usage is 50 per cent on the highway, it is sufficient to clear your DPF, so you should not have any issue with DPF clogging. 

If you like power and performance, the best diesel for your budget is the Mahindra XUV700 2.2 diesel with a 6-speed AT. The XUV 700 lower MX variants will fit in your budget.

Also, you might want to wait for the Mahindra XUV 7X0, which will be launched in early January, which is essentially a refreshed version of the XUV700 and with the same 2.2 diesel. 

Also, the 10-year limit is only in the Delhi area and is likely to be relaxed for the latest BS6.2 cars