What I did to install Debian Linux (Jessie) on Thinkpad X250:

Booting from USB device (to install Debian) was some challenge: in particular USB 3 needed to be disabled in BIOS (maybe some more BIOS tweaks that I cannot remember anymore).

To make the Trackpoint keys work:

In BIOS, disable Touchpad (anyway a good idea to prevent accidental touches there).

Added file /etc/modprobe.d/x250.conf with content
options psmouse proto=imps

Added file /usr/share/X11/xorg.conf.d/20-thinkpad.conf with content (works only if Touchpad is disabled in BIOS)

Section "InputClass"
Identifier "Trackpoint Wheel Emulation"
MatchProduct "PPS/2 IBM TrackPoint|DualPoint Stick|Synaptics Inc. Composite TouchPad / TrackPoint|ThinkPad USB Keyboard with TrackPoint|USB Trackpoint pointing device|Composite TouchPad / TrackPoint|PS/2 Synaptics TouchPad"
MatchDevicePath "/dev/input/event*"
Option "EmulateWheel" "true"
Option "EmulateWheelButton" "2"
Option "Emulate3Buttons" "false"
Option "XAxisMapping" "6 7"
Option "YAxisMapping" "4 5"
EndSection


Also to make side button of my Logitech USB mouse act as middle button:
Added file 20-logitech-mouse-side-button.conf with content

Section "InputClass"
Identifier "Logitech mouse side button remap"
MatchProduct "Logitech USB Receiver"
MatchDevicePath "/dev/input/event*"
Option "ButtonMapping" "1 0 3 4 5 6 7 2 9 10"
EndSection


(Still sometimes Logitech mouse stops completely to work, then unplugging USB receiver from docking station works -- still need to investigate that. Update it seems that plugging in the USB receiver into another USB port (=other USB type) helps.)

I also experience sometimes that my external Dell monitor connected via DP cable and my dock sometimes blanks for half a second: a firmware update of the dock is needed, but is only available as MS Windows executable. Any hints welcome how to do this via Linux! (A BIOS update via Linux is possible and worked.)
I do not have that problem when using the DVI-D port and cable of the dock -- however for 4k resolution, DP is better than DVI!

I also had an old 1440x900 display that did not report its native resolution when connected via VGA (which btw. reports as DP2). While I might probably add some modeline to some xconfig file as I last did probably 10 years ago, I did the following:

cvt 1440 900
Then pasted the modeline generated by cvt:
xrandr --output DP2 --newmode "1440x900_60.00" 106.50 1440 1528 1672 1904 900 903 909 934 -hsync +vsync
xrandr --addmode DP2 "1440x900"
xrandr --output DP2 --mode 1440x900


Also my other display sometimes gets no recognised:

cvt 1920 1080
Then pasted the modeline generated by cvt:
xrandr --output DP2 --newmode "1920x1080_60.00" 173.00 1920 2048 2248 2576 1080 1083 1088 1120 -hsync +vsync
xrandr --addmode DP2 "1920x1080"
xrandr --output DP2 --mode 1920x1080

For getting cloned display output with KDE "Display and Monitor" configuration system setting pane, the two screens have to dragged onto each other. However, I like
the old "Size & Orientation" pane more which can be obtained by installing the kde-workspace-randr package.

Just as reminder for me: to use Gutenprint for the photoprinter: create first in CUPS (e.g. via web interface) an entry for the photoprinter so that the printer gets an own queue. Then, in Gimp, this queue can be used when setting up the photoprinter there. In case the Print with Gutenprint menu entry does not show up in Gimp, an extra package needs to be installed: IIRC for Debian it is package: gimp-gutenprint