Yes, old computers are great to repurpose for projects like this. The advantage of the Arduino or the Pi is that there are a slew of accessories for projects like this.
Regarding the adhd, as a retired high school teacher, I am quite familiar with that challenge. One of the accommodations that helps is to identify the “getting stuck” early and walk away from the project before the frustration that tends to compromise the motivation sets in . Re-entering the project that was left at that emotional level always threatens to reignite it.
The last project I did with my Pi was to install NGINX (web server), PHP (programming language), and MySQL (database server) on it, a Wi-Fi dongle and an external hard drive to act as a web server on my home network. I have all of my digital music, old time and new time radio shows, and, lately, all my film since 1973 scanned. I wrote custom software that lets me search and play or display the results. Not so much Rube Goldberg except the software which my son, the programmer, rolls his eyes over when he sees how I do it. But, hey, it works.