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Hello,If I had it to start over again I would definitely consider the ESP8266.I suggest looking for level shifter and perhaps combining with a multiplexer like this article combined a 74HC4051 with a level shifter circuit (http://internetofhomethings.com/homethings/?p=530). The multiplexer would allow you to add the pH and temperature sensors and more. It sounds like you have a solution for the temperature sensor already; I found a waterproof temperature sensor on Amazon that I plan to incorporate but have yet to get around to it.Good luck.
Sounds great, I look forward to seeing an instructable on that.