Hardware Choices

For the Arduino I think I good place to go is Arduino starter packs. They contain the board itself as well as loads of other stuff that I'd probably end up having to buy or root around for at uni. These include basic things like wires, USBA/B cable, bread boards etc to more specific bits of kit which nevertheless would be useful for experiments such as transistors, resistors, LEDS, motors, photodiodes, etc. One UK based company that provides a good choice is Oomlout, offering a range of Arduino products here. They offer an all inclusive kit here and a slightly toned down version here. I'd probably opt for the latter since its cheaper but includes all the main components.

UPDATE

After borrowing an Arduino Diecimila I think the Uno would be better. Its very similar but one new feature is that it automatically selects where to draw power from (USB or Jack) based on what its plugged into. This would therefore be one less thing a user would need to know about when playing the instrument.

This piezo seems to be a good choice, though it is more expensive than others. This may be indicative of a better quality and since I hope to use the raw audio signal to not only trigger sounds but to influence them based on its characteristics, quality seems important. The piezo presents a reasonable frequency response (as reasonable as a piezo mic could be I guess) in the required range for knocks (100Hz to 5kHz).

This is a nicely sized square FSR from UK based Cool Components. I think a bigger size for the FSR, rather than the much smaller circular models, is a good idea since i think it would be easier to use ergonomically if the player didn't have to aim too much to use it could just lunge in its general direction without too many problems. It is important to note however that the FSR will need to form one resistor in a voltage divider in order to convert the resistance change of the FSR to a voltage change for input the arduino analogues.

I keep coming back to this loudspeaker driver so I suppose its appropriate. It certainly seems that way.