There are a few tips around the web for turning an old laptop into a distraction-free typewriter. You could go back to some of the very earliest laptops that ran DOS, but they are big and clunky and had poor battery life, and that was back when you could find batteries for them. Also, it’s handy to have at least one USB connector for moving text into the modern world.
The best guide I’ve seen ran on pcworld.com back in 2012. (when the photos were intact). It recommends using a ThinkPad 600 for its profile and excellent keyboard and has a reasonable walkthrough for how to set things up using Ubuntu Server. But why can’t someone create this as a disk image to make it even easier for folks? A Linux-derived TypewriterOS? (No, not this.)