Hi Cy,
Most of us are using the ESP8266 as is. There is a library to allow you to translate your sketches and flash them to the ESP directly. See my HowTo Esp8266 - #20 by kreggly
The ESP modules are not supported yet by Cayenne, however, many of us in the community are gleefully using them (follow the HowTo).
@Ian do you know if anyone has made the Huzzah board work as a wifi slave with the NodeMCU AT command set? There is code to do this in the Blynk library, through the serial port, I’ve just never tried it.
Now there is enough IO on the ESP-12 for you to drive your relays:
There is only one ADC input though. It works up to 3.3V on the ESP-12s, but there can be only one (Highlander reference, if you’re not with the lingo).
You can certainly get the I2C and SPI interfaces going though and chose your sensor.
This would put you into a fairly compact device for under 20 bucks by my estimates.
Craig