Just a thought... you could get a portable monitor, which are regularly available for around $50 for a 15.6" one, and pair it with a cheap mini pc or even a RaspberryPi. The Pi would even have the advantage of having GPIO pins so you could add buttons easily and cheaply. Since it's just going to be sat on your organ, I suspect it'd be fine to plug it in somewhere, but you could run it off a cheap battery pack if need be. A bit more setup work, but the result would likely be better than any tablet for your purposes.
You're off by a lot. My setup shows two pages at once, which means it's a 26-inch Android tablet (IIRC).
- To get integrated storage on the main board (for reliability), you'd have to go with something a lot less supported, like Rock Pi, and you'd spend over a hundred bucks just for the board.
- You'd have to add a 26-inch touchscreen monitor for another $250.
- You'd have to spend at least $20 on a case.
So even before the cables, you're at about $370 plus shipping, which is really close to the $400-ish price of the prebuilt Android tablet. That's with Android pre-installed, zero extra setup needed, no flash card to get accidentally dislodged and crash everything, just plug in, connect to a network, set a passcode if desired, and you're done.