Managed to install the game and the Mod, it works!
It is not possible to install the game on the C-drive, the standard directory (Programs (x86) cannot be removed, so I put it on another partition.
Before that I had to uninstall the game in the Programs (x86) directory.
Maybe, when you first install a Steam game, it is possible to choose a different standard directory.
Installing is in fact easy if you know what to do, but the correct procedure is hard to find. You have to read tutorials, threads, watch YouTube and start a thread yourself
So, right now I'm in my first campaign mission (TMO Atlantic Fleet, Narwhal), heading to Japan
Thanks for the advice, really appreciate it.