Downloads app is just an app that shows a history/list of everything you've recently downloaded (e.g. in the downloads folder). I prefer to use ESFileExplorer as it's more versatile as a file manager, and I don't care for an app that tracks file downloads/history.
In previous android versions, I believe the Media app was part of what cached/listed files such as images (thumbnails), music files, docs, etc. I vaguely recall disabling it in the past to find that all my MP3's and photos no longer appeared in the gallery/google music app. I would not disable it if my memory serves me correctly.
Yeah, your battery usage looks about in-par with mine. I came from a GNex and was used to seeing display always overtaking androidOS/systemUI (man, that phone had atrocious battery life and screen-on time)... the GS5 has such a great power-sipping display that I came to the same surprise as you to find it ranks on-par/sometimes below OS usage in a normal day. The battery life on this phone rocks.
EDIT: Media usage will sometimes spike if you've recently added a bunch of new files (photos, mp3's, etc.) to your device, as it's scanning/caching/indexing lists to the files. I'd give it a day or two if that's the case, and you'll see it won't consume much of anything once things are static on your phone.
