A short demo of my setup as it stands now.
The printer used is my trusty ancient HP5N connected to a RaspberryPi 3B via a parallel-to-USB adapter. The Pi is running CUPS and Sane.d and also some hacks that power on my antique laser printer which was covered in THIS project on Hackaday.io.
The scanner is a Canon LIDE 220. A small budget unit that conveniently is powered via USB. The buttons do not work because the button-daemon interfered with the network sharing. The front-end software however does all we need it to do.
First I use the front end to scan a colour document directly to preview.app and then I scan a document in black and white directly to the printer.