For 4 way games - I have 3 buttons. Most games want just 1 button, but a few require 2 buttons (like astro blaster) and only 1 4 way game I play requires 3 - Punch Out and Super Punchout.
Spinner games - this is a tougher question. By spinner games are you talking like Tempest, Arkanoid or do you also want to play driving games. Games like Tempest and Arkanoid use 1-2 buttons. Driving games will require a lot more buttons.
Trackball - I think 3 buttons is enough. I'm curious if anyone things you'll need more.
Driving games would be nice too, but mostly stuff like Ironman Stewart Super Off Road, which is one button I think, etc... So I would be safe by going 2 there (gas/brake) or 4 (gas/brake/shiftup/shiftdown)
But that's a lot of buttons for 1 player panel...
Let's see.
3 standard buttons for 4-way
3 atari style buttons for trackball
4 buttons for spinner.
I could combine these 4 with the 3 for the 4-way (so have only 4 buttons, depending on the layout...
so 7 buttons, plus the 3 controllers... Unless I cover all the bases and put 8 buttons total, just in case

The I-Pac can take it so...
How's this layout?
4-way on the left, trackball center, spinner on the right.

Marc