That's what I've linked to on previous page:
http://wavebeam.blogspot.com/2016/02...hat-plays.html
It's a really good guide, I recommend to read it all, but the link to a cable schematic is there too. Personally I can't solder so I went and bought a VGA-SCART converter from ArcadeForge, it does the same job.
Yeah, Wii is great for easy RGB output, have one too, problem is its a bit underpowered so MAME does not work well, also there's no microcomputers realy. I have Rpi with Pi2SCART RGB hat which is better.
PC is of course most powerful but the fact you need to make modelines for all different machines is a PITA...and I'm not sure it always works. Anyway, just getting into this myself, need to build a new, more modern box (win10 capable) because crtemudriver does not support XP anymore.