I don't to spend even $25 if the results are going to be unusable
You need some experience with the probes in order to judge. Buy the kit suggested in reply #2 (which comes with pH calibration solutions), stick it in the cooling water and see how the calibration changes over time. That is cheap education! Temperature compensation is required if the temperature changes.
Needless to say, your client won't be impressed if the device you build does not work.