I just received the 5 inch HDMI display this afternoon and based on its design I am going to modify my original plan to make the hardware implementation cleaner. The display was designed to mount directly on top of the GPIO pins of the RPi B and there is also a small PCB that jumpers the HDMI connectors together (the HDMI connectors are aligned vertically when the GPIO connector is plugged in). The GPIO header on the display uses on the first 26 pins so that it can be used with earlier RPi Bs. The display will plug onto the GPIO pins of the RPi0W, but I'm having a difficult time finding a good solution (short cable and/or adapters) to connect the HDMI connector of the display to the mini HDMI connector of the RPi0W. The short (6 inch) adapter cable that I normally use is stiff enough that it puts a lot of strain on the connectors. So, I've decided to just go ahead and use a RPi3B instead of a RPi0W. I have spares around because I have been upgrading some of my projects to the 3B+.https://showbox.bio/
There is still a minor issue in that the display header basically blocks the use of the first 26 GPIO pins. I should be able to get a flat ribbon connector down on the remaining 14 pins, so I'll just use the GPIOs at the end to control the relays. I'll need to figure out how I want to get the 5V power and also the I2C pins and 3.3V for the RTC in Phase Two. The display board does break out the 26 pins to some surface mount pads, so it may be as simple as finding the appropriate surface mount headers.