An important thing to consider when designing a thing like this is cost.
Lets make a little calculation:

- EFIKA 100$
- Touchscreen (8, 10 or 12 inch to be decided) around 150$
- Flash card (512MB) 30$
- Battery (4000mAh, 12V) 40$
- GFX card 50$
- Additional (wires, mounting, riser slot, etc.) 30$
-----------------------------------------------------------
- Total: 400$ ---> Expensive in my opinion...

- Add the Casing... expensive if you only make one!!

But negligable in large quantity.
And this is where it gets interesting because as soon as you have a quantity, the cost of this device can be driven down substantially.