1: Like I said, buy this cable and swap between GC and SNES + the cables for your other consoles. This would be my favourite “under 200€” option. I think it all depends on what you prefer. Watch this Video, you will see some SNES and 480i PS2 footage. If you like what you see, get the RT2X.
2: I think I know what you misunderstand: You don’t need to modify any of your consoles, the only problem is the N64. All other consoles just need the right cables. If there wasn’t the N64, everybody would have straight up recommended the OSSC. Like nmalinoski wrote, the N64 only outputs S-Video and Composite, which the OSSC doesn’t support. This is why he recommended the RT2X for transcoding the unmodded N64. The RT2X has these inputs.
The best “pricey” option would be: OSSC, RGB/Component cables and modded N64 (RGB or HDMI) – but you mentioned that you don’t want to mod your consoles. Also this will cost you more than 300€.
Does it mean that using OSSC + above mentioned cables (outputting RGB) will give me the best quality?
Yes. The OSSC will give you better image quality than the RT2X – but only if your TV supports the higher line multiplication modes. In worst case you have RT2X quality on an OSSC. But still OSSC has more options and 480p support – the latter is important for some PS2 games. There is no perfect solution.