@Comic-Sans Hey - bit late to the party here, but incase you (or anyone reading in future) swings by...
I'm making an arcade machine of my own (albeit mini). After a first proto (enclosure for a 5" display and USB ports for a few retro USB controllers) I decided to make a slightly larger one (7in screen this time) - and I also wanted arcade buttons.

I got the EG Plays ones (specifically these: https://www.amazon.co.uk/dp/B075DFNK24 ) and I've been really pleased with them. The stick is sturdy and feels really nice. The buttons are perhaps a little cheaper looking than some, but they're really responsive too and look really nice when lit up.

That's not to say the Sanwa ones (which I don't know much about) wouldn't be noticeable better, but at the end of the day I see many of these things as things that could be upgraded. The dimensions etc appear to be somewhat standard across both brands (and others). The type of person who builds an arcade cab is likely a hobbyist - so I'm sure the EG buttons won't go to waste if you replace them in future with better ones :)

There's a single encoder board which everything plugs into (with extra connections for other bits, etc) and this goes straight out via USB. I had these all plugged in and tested in my computer within 5 minutes of opening the box.

Good luck!

@mitu Many thanks for the quick feedback!

I have followed the advice on the network interface and the use of triggerhappy. I also looked into overlays in RetroArch and developed a solution. I would like to briefly explain this so that other users can also benefit from it.

First, I configured RetroArch so that content-specific overrides are loaded (if available). For each game for which an infoscreen is to be displayed, I have defined two overlay-pictures in the override. The first image is always a blank image (blank.png) and the second image shows the controls (e.g. pacman.png).

In the background, triggerhappy queries the keystroke of an unassigned button and then sends the following commands to RetroArch: OVERLAY_NEXT and PAUSE_TOGGLE. I had to program a small script to execute both commands simultaneously. Finally, I integrated the start of triggerhappy as a daemon into the boot process. Works great! :-)

If anyone has any questions or needs help, I will be happy to provide further information.

@j-e-f-f

That sounds like a pretty good plan. If you do a clean install from the recovery partition youll either get Lion or Snow Leopard in which either one would be sufficient as well as small.
If it currently has High Sierra installed you would have likely gotten an EFI update which allows for internet recovery and it may force you into high sierra as well, but it really doesn't matter which you use aside from space. You can also disconnect the ethernet cord and it -should- boot from the recovery partition instead giving you an older version of OSX.