Parts:

• Full Sized Guitar (as cheap as possible, it is only used as a case, you could also build a case yourself)
• Arduino (I chose the Mega, for its memory, but with the added SD-Card i think it could be downsized)
• Fluxamasynth Board (a midi synthesizer for arduino)
• Rechargable speaker (with a built-in amplifier and a 3.5mm jack. I bought a round one that covers the hole in the guitar)
• OLED-isplay
• 2 matrix-keyboards 12 keys
• SD-Card reader with SPI-nterface
• MCP 3008 A/D Chip with SPI-nterface
• 3 Potentiometers (for volume, speed and "raking")
• USB connectors m/f (to bring the arduino USB port to the case wall)
• Power connectors m/f (to bring the power jack to the case wall)
• 9V Battery compartement
• some wire (i used flat cable wherever possible)

Tip: I bought a new guitar for as low as 20 Euro, which had the advantage that it is only made of cottonwood, which could be easily cut wit a box cutter.