I will be using Plex to play most of my videos. Is there a free codecs out that will integrate well and allow me to play uncompressed untranscoded MKV files and DVD.ISO files on my client machines. It would be nice if there a free codec out to play BD ISO, but to the best of my knowledge one does not exist.
Plex doesn't require any extra codecs to play uncompressed MKV files. In fact there's no way I know to add extra codecs to Plex even if you wanted to. It won't however play HD audio (DTS-HD and TrueHD).
ISO's are actually a container, not a codec. I'm not entirely sure whether Plex can handle DVD isos.