I finally finished my disk sander, and it works quite well. I bought the motor on eBay for 19€, and spent another 40€ for the rest, so total cost was 60€. I'd say it was worth it from a cost point of view.
I didn't take any photos during the construction, but it's put together with screws, so I took it apart to take a few photos of the inside. As you can see, the construction is very simple and not very time consuming. The blue disk is an adhesive backed Velcro for Velcro sand paper disks. Initially, I tested the sander with just originally sandpaper glued on the disks