Looks like a great project! I can chime in on the LEDs. 120mm / 8 is 15mm LED spacing. If you can live with 16.67mm spacing, either by dropping an LED or expanding the diameter to 134mm, then the new WS2811/WS2812 pixels would be pretty much ideal. I just put some in my store, but you can probably locate them elsewhere: http://macetech.com/store/index.php?main_page=product_info&cPath=1&products_id=44
That strip is spaced at 16.67mm between LEDs. Each LED is 5x5mm, and a couple mm tall. Every LED can be addressed separately, and the strips can be cut at any point. So you could do 8 across, 6 across, 4 across, etc in order to fit within your round platform. Only 8 strips needed.
Actually constructing the platform could be fun. It would be ideal to take a layered approach. First, a white plastic layer for the LED strips to be glued on. Then, an opaque white layer with laser cut square holes, leaving thin lightproof barriers between each pixel. Maybe 1/4 or 3/8 thick. Then, a layer of translucent white plastic; perhaps 40% or 60% sign-white, 1/8" thick. Below that, the base could be made thick enough to hold electronics and batteries (AA probably best choice, but a flat Lithium-Ion could work).
I'm assuming you'd need some wires leading up to the disco ball motor. A small channel for that could be built into the design, along with laser cut holes to match pins on the feet of the model and allow easy gluing.
A little tougher, but the same type of LEDs could be installed in several locations on the model itself.
If all this is daunting...the project looks fun enough that I'd be willing to help out with laser cutting and constructing the LED base at least.