when adding tunes to your collection you have an option of either copying them to a new directory or using the existing directory as your itunes library. i know itunes likes to sort items in folders such as this:
/artist/album/song.ext & /artist/album/albumart.ext
if your directory is already set up like this (which most are) then it shouldn't modify how your directory is set up. in the settings you can change what file type is default for new music (in case you expand using itunes later).
no, you shouldnt require additional store, but backing up your collection somehow once in awhile is highly suggested. its not uncommon for hard drives to fail at any time.
you can play the songs from the same directory. itunes and winamp can...