OK thanks for clarifying, I am still thinking about magnetic discs.
I read that amanda seems to be a pretty descent tool?
I will follow your suggestion and go with an hdd and do daily backups, now I need to get this SD card running with the least amount of writes.
I’ve got a 5V 6A power supply and will attach an USB hdd and do backups with amanda and probably one dd every couple of weeks.
As UPS I will look into getting a power-bank.
See here where I am stuck at the moment