i'd recommend gummo, i think. it's... not that great a film, but it's worth seeing. i don't know. it's a lot to bear, emotionally and spiritually (and even physically; you'll know what i mean when the bathroom scene happens). it's also probably the most masturbatory film ever created, and probably isn't nearly as aesthetically meritorious as korine thinks it is.
that said, it has one of the most beautiful and heart-wrenching endings of any movie i've ever seen, so it's worth sticking through it until the end.
yeah, whether or not the non-actors-as-actors thing works really varies from piece to piece, and i can't even put my finger on why it works when it does. it works really well in harmony korine's gummo, i'd say, and its best application that i've seen is probably werner herzog's enigma of kaspar hauser, but that partially stems from its absurdity (dude is playing someone who is supposed to be 25 years his junior lol).