ALLNET banana pi board BPi-M3 Octa-Core CPU - is there software-support for this 32 bit version
in other words - can i get a 32 bit version at Debian
or does debian only support the 64 bit version of the follow up board!°?
Debian does not support any of those boards. You can find one Debian from board maker and there is some test version from Armbian with a modern 4.17.y kernel. https://dl.armbian.com/bananapim3/Debian_stretch_next.7z
What is diff between Armbian and Debian. In short: https://forum.armbian.com/topic/7787-what-difference-between-armium-and-debian-linux/
M3 has 32bit SoC.