This is basically a hydraulic ram. I would suggest using an acme screw or ballscrew rather than a rack and pinion. You will get a much higher output force from a given input motor torque. And the higher the screw pitch the better. You cannot answer the question of how much torque you need until you can determine how much output force you need. Then it is a simple matter of choosing a gear reduction ratios to get the motor speed and torque you will require.
The only way to measure the force required is to apply known forces in increments until you achieve the flow you need. You could use objects of a known weight and work out some kind of connection arrangement, direct or lever or pulley.