I think it was suggested before, somewhere.
It seems like hardware is the main moneyburner here.

Why invent it again?
Use some small device with a bit cpu power, a good enough display, battery, usb connection and buttons.
Sounds familiar?
--> MP3 Player <--

Now I didn't research into this much. However, I found http://www.rockbox.org/ which replaces the stock firmware on a lot of mp3 players. We may use it directly, or parts of it, or just use it as a pointer to see which mp3 players are usable.

How about "sandisk clip":


Or "SanDisk Sansa c200":


edit:
even much more simple and surely cheaper, not based on rockbox:
http://www.s1mp3.org/en/docs_userguide.php

I would prefer one of the first two players..

I would use one of those. And buy a reprogrammed one, or maybe do it myself and send money in the direction of the firmware programmers.

Now these are strictly offline. No Paypal possible on these, as someone shouted on the original vapor-Ellet.. But good enough to sign transactions, for sure!

So, why are people burning truckloads of money on re-inventing the hardware? It's all in the software anyway!

Ente