@nbelles: there is NO ‘precompiled’ firmware for OSPi: you install it and compile it directly. Follow the instructions here:
https://openthings.freshdesk.com/a/solutions/articles/5000631599-installing-and-updating-the-unified-firmware
I assume you are referring to the unified firmware. If you want to install any third-party firmware, you have to follow their instructions.
@WildBillpdx: yes, I would definitely vote for RPi Zero W H — the lowest cost RPi with built-in WiFi.