Dear MartinC,
For pH, go to Section 10H of the draft copy of my maker e-textbook at
https://fit.instructure.com/files/48078947/download?download_frd=1
starting on p. 332.
https://www.ebay.com/itm/Liquid-PH-0-14-Value-Detect-Sensor-Module-PH-Electrode-Probe-BNC-for-Arduino/201758085638 is the cheapest source for pH probes and shields.
I order from them in bulk (60 usually) every two years for my class. DFRobot is plan B. Atlas Scientific is way too expensive, although I started with them about 6 years ago.
My class is available at
https://fit.instructure.com/courses/629658
The section after pH is for salinity (electrical conductivity) testing. For my research, I use a more expensive EC, a DO, and an ORP sensor as well
Most of the kit parts are linked to on p. 293. If you need more detail, just post more to the forum.
Best regards,
Jim Brenner