You do know you can add the controllers together to create a larger controller. So choose what is more important for present. Then later on you can get another controller a buss cable and a terminator. Then you can create a collective with the two. They then share inputs and outputs so it acts like a single controller with all the inputs and outputs of both. The second controller can be controller only with the terminator and cable. It does not have to be a starter kit. The first one get a starter kit. I currently have 8 control units in a collective. I have some information on the Hydros in my build thread starting at post #119. The first one I got was a X4, but I needed the 0-10v inputs for a specific purpose. I didn't need the 2 pH ports on it for what I was going to use it for. I have since added various control units. For ATO use a drive port or an output on an XP8. The wifi outputs will stay in the current state if communications is lost with the controller. Not sure that would be good if the ATO was on when that happened. I use drive ports for all my dosing pumps including ATW and AWC. I also use the to drive the solenoids on my RODI unit to refill the storage tanks when they go low.
Click on my build thread banner to get to the build thread where I have the Hydros info starting at post #119.