Hi @piki.
Welcome to the Forum where you will find many helpful and friendly people (and also technically competent 
)
Check out this Post Multiroom Audio in Spanish Villa
Let’s Start with the Main System:
I would recommend the PRO DIY Board (equipped with WiFi & Wired LAN Port. The MINI is WiFi only. For me the cost difference is negligible and unless you are limited with size I Would suggest the PRO and if you have an Ethernet Switch/Router close by I would also recommend Wired Ethernet (if possible).
Moving on to the passive speakers in another room
I would recommend the UPSTREAM2 AMP V4 DIY Board. I would also recommend Wired Ethernet (if possible), however it will work also purely on WiFi.
Regarding Architecture
These products communicate on TCP/IP Network via your Home Router. No Additional HUB (e.g. Sonos) is required So the beauty of the Design is it doesn’t matter if you have 1,2,3… etc. devices the comms will take place through your existing router.
Hope this helps
Kevin