whizbo pointed out that you can buy Kindle Paperwhite devices on ebay cheap that are locked in “demo” mode. But you can open it up, get access to the serial, and reinstall it with stock firmware.

The serial on the PW uses 1.8v.

Kindle and EPUB

Hardware

I ordered 4 on ebay and got them, they have “kindle” printed on the back which is supposed to indicate paperwhite 1 (vs “amazon” for paperwhite 2).
They appear to have 3g modems and after I had them turned on for a bit they all updated their firmware to a newer version of the demo firmware!
lsusb:

Bus 001 Device 116: ID 1949:0004 Lab126, Inc. Amazon Kindle 3/4/Paperwhite

Update process

Battery