Could you just never connect the kindle to wifi? I'm pretty sure you can just load supported formats over USB via mass storage mode so there's no way it would phone home. Unless I've missed something obvious.
It's probably easier than you think. He printed instructions on the pcb so you can't miss them lol. I'm sure you can find people on ebay selling completed boards
I can't imagine they have bulletproof security or anything, but any of the Kobo e-readers are easy to load up with PDFs/epubs on your own via USB.
I use ad-supported (so cheaper) kindles I jailbreak and sanitize myself - all the ads and stuff that could call home to amazon is removed, and a new reader is installed. I also never go online with it for added security (don't trust much a browser that's rarely updated in terms of 0days), but if it was online it wouldn't even try to reach amazon, or fail trying to. It's not that complex to do if you get an UART cable and open the kindle up; if there's enough interest I might write a guide for it ?
Also, with that approach you can install any app you want - games, SSH clients and so on; it's a surprisingly close to vanilla Linux system.