There are some answers considering the installation of the cuda drivers on debian 9 (stretch) here, however as far as I can judge, they mix Ubuntu with Debian sources, and this probably crashed my system when I followed their instructions.
There is a tutorial on the official Debian site how to install it without creating a "FrankenDebian" (this is how they call it), but it is for Debian 8 (Jessy).
https://wiki.debian.org/NvidiaGraphicsDrivers
I do not know enough, to understand how I should adjust this tutorial to the needs of Debian 9 (stretch)