What does this iMac support and can I update the Fusion Drive to an SSD?

I’m getting this iMac as a hand me down, not really need it to be super fast and snappy (or my main computer) but would like to use it for either macOS or Debian.

What OS will this support and can I upgrade the drive to an SSD? Or even upgrade the ram? I have some 2.5” ssds that I don’t use.