I will be using the Honeywell RLV4300A as my base unit. There are two circuit boards in this device, one high voltage board and a low voltage interface and control board. These are connected using a 4 pin header. No modifications should be needed to the high voltage board, which is good because I don't want to play with that. The existing low voltage board currently utilizes a Atmel ATmega169P processor, to control the power, display and 8 buttons.
My overall goal will be to replace the low voltage board with a custom design. This custom design should use the ESP8266 as the main processor, with custom code to emulate the existing functionality and enable remote control and configuration.