Yes, Redmi 6 is a VoLTE phone.
Yes, Redmi 6 does have a dedicated MicroSD card slot.
Yes, Redmi 6 does have the ambient light sensor.
Redmi 6 is made of industrial grade polycarbonate material.
If your budget absolutely can’t exceed the Rs. 8,000 mark, go with the Redmi 6 without any concerns whatsoever.