MC has all of it's video support "baked in". There are no CODECS to install or anything like that. In practice I've had a small handful of videos that would crash MC and not import. I believe those videos had really old unusual CODECs or perhaps were authored in a non-standard way.
I have never had an MP4 video that would not import.
As far as streaming to a DLNA host, that is outside the scope of what MC will import and what MC will play. The DLNA host and MC's configuration for DLNA video are separate. Once you get the importing, and local playing, working, maybe you will start another thread in the Media Network forum about DLNA video support.
Finally, I have had a small number of videos import as "document" type. I'm not clear on why this happens, but it seems to mostly be when I'm importing lots of videos. But not always.
Luckily the cure is very very simple. Just select the "document" and open the tagging window (Edit > Tag). Then find the Media Type and set it to Video. You may also need to set the Media Sub Type to Movie, TV Show, etc. In EVERY case on my system, simply changing these two fields has yielded video that I can play and that appears in the proper place in MC.
Good luck.
Brian.