Seriously, import a couple of Japanese KOCs and hack them to shreds. Unlike the US versions, these don't usually die for no reason. They can be had for about $70 each, which is less than you're considering spending on al this gear and they do EXACTLY what you want. As a bonus, they even include the optics you need, and a handy wiring pigtail for all your buttons (including start and select, which bring your total up to 10 buttons per player plus the two optical inputs). Heck, they even include a correctly made toothed disk for the optics, if you want to use it.
I don't normally advocate hacking up console controllers, so if that's what i'm suggesting it's a safe bet that it's the easiest and cheapest route. If you want USB for sims, just grab a cheap playstation to USB converter. This is seriously how almost ALL 'professional' IIDX ASCs are built. It's not how I would build it, but since you don't want to build it the way the actual machine is built, this is probably the best way.
To answer the other quesiton that came up, "spinners" don't normally "work" on the PS2. Support is up to the game, but the way IIDX is set up, clockwise rotation is mapped to the up arrow on the D-Pad, and counterclockwise is mapped to the Down arrow (yes, this is slightly not what you'd expect). If you continue to rotate, it just continues to say that the button is pressed.