I will make some high-level comments: in general if you are looking to build a custom system, OpenSprinkler would be a pretty suitable platform, due to its open-source nature and the availability of spare pins which you can use to extend the hardware functionality. A while back I modified an OpenSprinkler to become a biology experiment controller:
http://rayshobby.net/modified-opensprinkler-becomes-a-biology-experiment-controller-2/
it controls two linear actuators, two solid state relays, one air valve and it monitors CO2 level using a CO2 sensor.
The existing OpenSprinkler firmware can probably cover most of your need already. You will likely have to change the code to support EC sensor. Because OpenSprinkler is based on the Arduino software platform, if you have any sensor in mind, you can just Google ‘EC sensor Arduino’ and you should find some links with useful information, perhaps even with an Arduino library or source code you can make use of directly.
This is just my high-level comments. I did take a quick look at your description of the system, and I am pretty sure it can all be done. The question is how much hardware and software changes it involves, hopefully not much.