It's actually pretty simple if you have a frame with horizontal dropouts and 132mm or 135mm spacing. You can respace the frame if it's from the 80s.

You can use the Alfine 8 without disk brakes, it has a rubber cover for the disk connector.

The Alfine 8 is only 134,37 € https://www.bike24.com/1.php?content...7C%7C%7C%7C%7C
You also need the Small parts package: https://www.bike24.com/1.php?content...7C%7C%7C%7C%7C
The rear sprocket https://www.bike24.com/1.php?content...7C%7C%7C%7C%7C

And a shifter of your choice: the Shimano trigger https://www.bike24.com/1.php?content...0,2,116;page=3
or the MicroShift bar-end (it'll require a cable adjuster) https://www.niagaracycle.com/categor...-nexus-inter-8

Then you have to get new spokes and relace the wheel.