>I'm quite confused about the whole D-sub / VGA / rgb stuff, so any help (ideally with links or images) would be highly appreciated. I also have an OSSC upscaler for vga and scart, so I hope to use that to stream with it.
This page should help. You can get an adapter like this one that will let you use a VGA cable, or you could make your own by cutting the end off a VGA cable and soldering a DA-15 connector to it.
The OSSC is happy to accept both 24 kHz and 31 kHz modes, though its VGA input doesn't have much in the way of filtering so it's possible you'll get some noise in the picture. My Ap2's video output is pretty clean, but I don't know about the older machines. If it's an issue, you could try using the SCART input instead, but you'll need a sync combiner.
>I'm located in the EU and I'm also wondering if I can just regular old pc power cables or if I need a converter or something.
You need a step-down transformer. The PC-98 power supply isn't compatible with 230V mains.
Project ANISE might be worth checking out:
https://tomyun.pe.kr/projectanise/
Description, translated by Google:
> There were games that burned the hearts of teenagers in the 90s, and they were "Japanese adventure" games made in Japan. Among them, Elf and its subsidiary Silky's were the most actively presenting works. All of their games used a common scripting engine, and analyzing and translating them into Korean was very popular at one time. The purpose of this project is to re-create that scripting engine so that it can run on a variety of platforms to suit today's environment. You could call it a kind of emulator.
(Project ANISE has not been in active development for quite a while, though.)
And another interesting piece of software might be: