I am running KDE Neon 5.12.1 on my ThinPad T61 which has a Nvidia nvs 140M graphics board. As I experienced a number system crashes/automatic reboots mostly when closing windows, I thought it might be a good idea to install the NVUDIA proprietary driver nvidia-340 plus the nvidia-settings. Installation via the 'restricted' ppa using 'sudo ubuntu-drivers autoinstall' worked fine, and using 'ubuntu-drivers list' confirms the driver is there. However, this command also finds the driver 'nvidia-304', which definitely is not there. Now, the main issue is the KDE Driver manager. When I start the driver manager, It seems to look for the driver for a while but then it quits from the search and nothing else happens. On my desktop PC, which runs kubuntu 17.10, the driver manager works perfectly.
After upgrading to Neon 5.12.2 I did a new clean install of 'nvidia-340' and 'nvidia-settings' using the 'ppa:graphics-drivers/ppa' and it worked well. However, the 'Driver Manager' still does not display anything after searching for proprietary drivers.