go to dipmicro and grab their collections of partshttp://www.dipmicro.com/store/index.php?page=5&act=viewCat&catId=470
or at least a bunch of 270 ohm resistors, 0.1uF caps, NPN transistors or ULN2803 driver chips (8 drive transistors).
Some 74HC595 shift out registers
5V power supply, with 5.5/2.1mm plug
some push button switcheshttp://www.dipmicro.com/store/index.php?act=viewCat&catId=548
Browse around, get some ideas, costs are really good and shipping is low.
Maybe a spare ATmega328 in case you damage one.
I don't see any 7-segment displays, maybe pick up a few of those (common anode) and maybe some LED matrixes?http://www.mpja.com/products.asp?dept=121