Is there any chance to get an MPG codec only for Windows 8 somewhere, instead of all those free multiple codec packages I don't trust so much?
Download Codec Installation Package. This package can be used as an alternative to automatically downloading Windows Media Codecs, or to correct problems experienced with previously-downloaded codecs. LINK.
*Install only codecs, filters, or plug-ins from trusted, authorized sources, such as the website of the official supplier. Even then, use caution: some codec suppliers offer minimal customer support. Before installing any digital media components, set a system restore point. The restore point enables you to return to your original system configuration, if necessary. And I think this link is trustable. If you are worried, just create a restore point before. LINK.
The Media Center pack for Windows 8 adds MPEG codecs to the operating system. There was a limited time offer when the upgrade was free, but not anymore. If the media center pack is installed, then Windows Store apps are able to use the MPEG codecs as well (not available on Windows RT).
The 'Codec Installation Package' linked in the answer above does not contain MPEG codecs - just Windows Media codecs (which are different from MPEG)
If you want a free solution, my recommendation is to get FFDShow: http://ffdshow-tryout.sourceforge.net/