I would add a unthroddle button to replace the reset. I have this to both unthrottle AND showfps both set to the same button. So when I press it, the game speeds up and shows the fps (so you know its going fast... sometimes its not obvious).
Anyway, I use that more then I use the pause button. Enough that I keep wanting to make it closer to the finalburn version that will only run when the button is pressed... but that would require modification to the input I don't know about. My final 'perfect' version would be while the button is pressed, it changes it to frameskip maxed, unthrottle and show the fps... Then when you release it goes back to the previous frameskip, throttles and hides the fps (if it hid it to begin with).
And agree with much of the above. But consider buying some cheap press board and building a temp control panel. Get it wired up and use it for a week or two... Then do the real one... Press board is super cheap... and you might change your mind... want extra buttons... find you never use others... find thta you want more space around the trackball for some game...ect...