1) You can load AVI files with AVISource or DSS (DirectShowSource) -- since DSS rather never really is good at certain things, I'd rather keep away from it if only possible (If whatever you saved your video with is directshow-only or if DSS gives you better output, then it's alright to use).
2) Damn, you were given links and examples... just that they missed ratecontrol (which you should decide by yourself -- here I'm using the 1pass CRF "constant quality" mode with a value 19, which is rather transparent on HD video and quite OK'ish SD, depending on your eyes and usedness to perfection. Higher value means "use less bitrate" and a lower one means "use more bitrate", so basically you can try finding the highest value that still gives you acceptable output and keep encoding with that.
Code:
x264.exe --crf 19 --preset slow --output "x:\path\x264output.mkv" "x:\path\avisynthscript.avs"
I usually write the input file name before the settings and use -o instead of --output for the output setup. I also would not recommend using old MeGUI profile settings in any case, which is what you seem to have found...
Also, you were linked with
this URL, which explains the presets, which is pretty much all that you need to touch when encoding. Tuning might come in later, but I'd say that mostly you can just use the preset to control the quality/speed compression ratio.