There are something you want to do DIY... those circuits are not one of them. Trust me. I have done quite a bit of research on these. Of course it is possible to create them yourself... however, the time and energy it would take is immense if you want to have verifiable precision results. These circuits are tricky and careful care needs to be taken in every step of the design. Also, the code on the chips to make them work properly, the wiring, electrical interference noise... etc... all of those things have to be taken into account. Salinity is an especially difficult circuit to design properly. A man's gotta know his limitations. There are PLENTY of other fun things you could be doing with a controller and monitor design than working out the nitty gritty details of those circuits. I myself wasted quite a bit of time trying to make my own. I had some degree of success but didn't want to take the time and expense that would have been necessary to build them to an acceptable tolerance of error. I realize your enthusiasm and desire to build your own. I respect that, but do consider my advice... just buy them and move on to more interesting and overall useful parts of the design.
i agree 1000000000% this is an interesting project but it is a waste of time if i were u id save urself the headache and just buy a Neptune systems Apex which has everything you need along with the option to infinitely expand the modules.