A quick google seems to suggest around 5-20mhz for GPIO on the rpi, not sure about interupts.
I am looking at a flow sensor here in Australia, here are the specs
Measurement range 1.0 – 60 LPM
Calibration Flow(LPM) = f(H2) x
23.8 (should be a divide by bar above the 23.8 so it looks like f(H2) x OVER 23.8)
Repeatability <= 1%
Accuracy <=2.5% of FULL SCALE
Protection class IP65
Max uid temperature 60 C
Max ambient temp range 0 C – +60 C
Max uid pressure 1.5 Mpa
Operating voltage +5V
Output frequency square wave
I’m unsure of what the (h2) and x is in the calibration. Would something like this work ok though?