So I installed QC today. Apart from turning the music off, adjust FOV, sens and key bindings, I haven't done much. As it is, the game is unplayable.

I don't know if to adjust for ping. Sometimes shots hit seemingly without delay, sometimes they go right through.
Enemy model visibility is absolute garbage.
Worst of all, the sound. Oh my god, the sound. It just sounds totally muffled and random. I have zero idea where my opponent is. Sometimes it even seems to come from the exact opposite direction.

Oh, and btw, the game gfx look completely different to what it looks like when I watch streams, no matter if I set gx to minimum or maximum. The gfx just look way different, and actually much better when I watch streams.

Another question - where do I set my FPS limit at? Is that even important, as in Q3 (-> constant 125 fps, otherwise limited jump height)?

Well, I suppose, don't bother. I remember starting Q3 demo up for the first time, which felt just right from the beginning, and just got better with config tuning etc. QC just feels wrong from the beginning. If a game doesn't feel right per default, there's probably no way to fix it.