I'm guessing you are looking at the "black" flow meters not the "grey" ones - which are rated to 80°C. I have been looking into this also (well not too much as other things have been taking over as late!).
My thoughts were firstly around using the flowmeter input to control an output pulse to the ball valve to open/closed to get the right flowrate, e.g. if the flow is to high start closing the valve in short pulses until the correct flow is achieved. I then started to look at controlling the motor with a VFD, but found that would be a pain for the type of motor we use and potentially you can get the same results from using PWM control of the pump (same control pilosophy as example above).
One other thing is I do agree that Kal's design is a good build, but it is good for what it is meant to be = a 3x PID hardwired control. What you want to do with the flow meters would likely be more advanced with an arduino that what is required to "clone" what Kal has done. I would not fully discount using the Arduino to control all of the brewery not just the flows.
Also the accuracy of the flow meters seem to be "out of the box" at 10%, but I have seen once they are calibrated they seem to be stable, although I think that was stable at a certain flow = you would need to calibrate them for a range of flows and then do a bit of post processing to get the corrected flow. Also the pulse per second are not linear with flowrate = post processing is again required if using various flowrates.
from the 1.5 - 25 LPM datasheet:
3.5 ± 1.0 pps @ 1.5 Liter/min. (+/- 29% and 2.33 Pulse per Second @ 1 LPM)
26.1 ± 3.0 pps @ 6.0 Liter/min. (+/- 11% and 4.35 PPS @ 1 LPM)
57.8 ± 5.0 pps @ 15.0 Liter/min. (+/- 9% and 3.85 PPS @ 1 LPM)
76.2 ± 7.0 pps @ 20.0 Liter/min. (+/- 9% and 3.81 PPS @ 1 LPM)