At the water heater end, requirements are minimal. Just about any ESP32 variant would fit the bill. I decided to use something from their Atom series since they are cheap and I already have some on hand. Specifically, I'll use an Atom Lite, which has two physical buttons and a single RGB LED. 

They have a couple different RS485 interface components that will work with the Atom series. The cheapest option is their RS485 to TTL Converter Unit. 

For a tiny amount more, their ATOM Tail485 - RS485 Converter for ATOM makes for a tidier package since it plugs directly into the Atom Lite.

In the end, I didn't choose either of those. I spent a few bucks more and chose the ATOMIC RS485 Base.

The reason for that choice is because it leaves the 4-pin port on the Atom Lite exposed. (It also leaves the USB-C port exposed for power, though once it's wired up to an RS485 circuit, that circuit can power everything.) I'm not sure if the reverse-engineered RS485 conversation will let me remotely "push the button" to call for hot water. For insurance, my first step will be to implement the hot button feature using a relay, as in the previous project. This Mini 3A Relay Unit interfaces to the Atom Lite via that 4-pin port.

The total cost for these three components is under US$20, which is not too bad for a one-off project.