How are you going to handle these two situations?
1) I think that you are going to get a ton of crap entries - and content that has been grabbed from copyrighted sources and reworked enough to pass duplicate filters. Look what people do here for a lousy link! They would quickly steal something and send it in to you. I say this from experience running small contests on my site that have prizes as small as a baseball cap.
2) Why should a really great writer enter this contest? If I write an article that might win a contest it is going to be much more valuable as a unique piece of content on my own website.
* "It's not the size of the dog in the fight that matters, it's the size of the fight in the dog." Mark Twain
* "Free advice isn't worth much. Cheap advice is worth even less." EGOL