I've thought about a button box a couple of times but not got around to doing anything yet, although personally I think the pre-built 'control boards' or even full boxes are way overpriced. I've got a usb gamepad here that I half-plan to repurpose into a button box, 'only' 14 digital inputs (4 axes as well in theory) but it cost £2.34 delivered from good ol' china
Anyway, looking at those commands, I use regularly:
brake bias (I've got it assigned to my G25 shifter in sequential mode, I always use paddles)
Enter/Exit/Tow car (one of the 2 G25 buttons)
Pit Limiter (one of the ones on the shifter pod)
But then I use the keyboard for the rest. For buttons I also have setup 2 voice chat buttons, one in-game the other for mumble, a mute for mumble, and the ignition/starter buttons on the shifter pod as well.
What you can do with button boxes (well just spare buttons in general) is use xpadder to create macros, e.g. for oval racing you can set left side tyres only for the pit stop, or no fuel, etc.