You shouldn't install gcc and libraries from source, only if they are not present in your distrinution repositories. So, install gcc and all development stuff from your distro packages. Otherwise, you'll have big problems in the future, even if you manage to install it from the source.
The Mplayer page describes what you should do with codecs. It's pretty well documented, actually. http://www.mplayerhq.hu
In short, you unpack your codecs into /usr/local/lib/codecs before compiling MPlayer and unpack you skins into /usr/local/share/mplayer/skins Name one of the skin directories "default".
As to the text editors, there's emacs, vi, nano, mcedit and many others.