... you can run installation cost and see what would be worth putting in - DP meter or ultrasonic
... you can download the calc prog from here
http://www.lavell.lnk.pl/str/soft/soft.html
... try to run simulation on Process Control Calculators from www.lavell.lnk.pl
or if the pressure is high you can use ultrasonic flowmeters... Panametrics came up with those.
I'm been working with ultrasonic flow meter... don't think there is a way to determine the water cut...
You can only measure 2-phase fluids but the volumetric or mass flow would be calculated as one.