Welcome aboard, Epetti.
Obligatory first question: Are you SURE you're going to use more than 2 player positions? (Lots of people who build a 4-player panel realize several years later that they almost never used P3/P4 and could have used Neutrik USB Feedthrus/USB gamepads for those rare occasions. YMMV.)
Have you sorted out which emulators (MAME, Visual Pinball, etc.) and front end software (MALA, Hyperspin, etc.) you want to use?
What admin buttons do you need for the emulators/front end?
What are the 4 buttons above the trackball?
Have you decided between Lono2 and I-Pac4? (assuming you're still wanting 4-player panel) Deconflicted the keymap?
Will you be trimming the front corners or leaving it a big rectangle?
What genre of games do you prefer? (May help you decide which sticks to use.)
Have you visited
Slagcoin to find the layout you like best? Definitely try layouts on cardboard mockup before the sawdust flies.
Plan it through before you order anything or dril any holes.
Scott