Maybe you could describe how you want your interface to work? It would make it easier to comment on the parts you would need. For example, why do you think you'll need so many leds? Are they just for status, or are you trying to use them for photosensors?
As for designing the circuits, I would start with drawing a block diagram based on functional blocks or units. Then for each functional block, you can define the actual circuitry needed. From here, you can determine things like curent & voltage requirements for the blocks, add up the blocks to get overall requirements.
As for where to get the parts, check out suppliers like Digikey, Jameco, Mouser, etc. Just google "electronic parts" and you'll find them. All of the big suppliers will send you free catalogs, which are nice to have on hand for ideas & inspiration.
Lastly, if you'll be dedicating an Arduino board to this project, you may want to use the Mega instead of the Duemilanove. It has a lot more expandability.