I installed HA Green [0] (and their Zigbee dongle) at my in-laws house, FIL is a non-IT smart guy and is happily managing and even adding his own stuff.
I've been using HA with a Pi, an SD card and a second hand zigbee dongle for the past 3 years. As long as you are able to read instructions to flash the SD card, you can use HA.
It always seems like you need $100+ dollars of hardware to run it well, not just a pi and an SD card.
It seems like it's a lot better now, but until recently it didn't seem like it was anywhere near PLC grade reliability.
Is it up to the level where you could install it at a non-technical users house and expect it to run for five years without maintenance?