I tried to assemble everything I'd need before I started too but ended up in analysis paralysis and found it best to just get started.
I've had to purchase more supplies about 20 times now, including:
paint (3 times), wireties, usb extension cables, molex connectors, usb -> ps2 adapter, extra screws, plastic washers, standoffs, a slot cutter for my router, plexiglass and polycarbonate cut to fit, a poster, wall corner protection strips, angle aluminum, a 1 1/8" drill bit, a flush cutting bit, some black posterboard...
Geez. I got tired of typing.
In all honesty, you can spend more time drawing up your plan than building the thing. Nothing wrong with a good plan, but even steel erectors and carpenters make changes on the fly.
I did some "engineering" at ACE hardware just yesterday.