Yes, Redmi 6 can support 2 4G SIM cards at the same time. It is a dual 4G phone. It also has VoLTE support.
Redmi 6 is made of industrial grade polycarbonate material.
Yes, Redmi 6 does have GPS support.
Yes, Redmi 6 does have the 3.5mm headphone jack.
If your budget absolutely can’t exceed the Rs. 8,000 mark, go with the Redmi 6 without any concerns whatsoever.