So bit of rant here but I don’t think you could do it cost effectively.
I work in Industrial Automation and looked into this. There is no way you could do what apex/GHL does for the same price. If you had 1000 other people who wanted it too so you could do bulk orders, then okay maybe. But to do a 1 off, no way.
The sensing of Conductivity, pH, Orp, would cost you $600 ish in probes and circuits alone. Then there is the EB8, simple to build with some digital IO, transistors, and beefy relays...but you are still looking at 75-100 in parts.
Also remember that you are able to control it from anywhere with internet access with no monthly fee. So basically you have to find a free cloud platform that you can talk to, or pay your IP provider for a public, static IP address. The latter is not cheap.
If you go buy the new apex base unit - 4 probes, brain, new EB8, it’s $800. I challenge anyone to build it for that hardware price (not even your time) using an industrial PLC or even a raspberry pie.