Harmy said:
Thanks guys. I am a total avisynth analphabet but I really want to do it right, so if avisynth is the way to go, I'll need a lot of help with that.
Well, as long as you don't do any filtering like resizing, debanding, sharpening etc. you don't need avisynth at all, as x264 can index any video that is playable by libav/ffmpeg on its own.