Is a buffer required for this gas flow sensor to ADC connection?
According to the datasheet the output is specified with a load of 10K\$\Omega\$. 

That implies a relatively low source resistance, probably well under 500 ohms, given the +-5% accuracy specification.

Most microcontroller ADCs are quite happy with a few k\$\Omega\$ source impedance. The chip you are intending to use is even more forgiving, if you look at the differential and common mode input impedances. 

Of course if you want to add signal conditioning such as low-pass filtering you may want a buffer, but chances are that's already built into the sensor.