I'm looking to conduct a contest on a standalone website (dedicated to just that purpose) and would like to see if anyone has suggestions for any existing solutions that will help me facilitate it as I'd like.

Here's a basic description of the contest and website that I have in mind (and some of the functionality I imagine I'd required from the site):

Contest: Visitors guess what the price of a stock will be on a future date. Person who has the closest guess wins a cash prize.

Website: Form for contest submission is prominent at top of the homepage. It will include about five custom fields, including the entrant's guess and text field for them to briefly explain rationale for guess. After entry is submitted, data is fed into a database, the entrant is redirected to a confirmation page and a new page on the site is automatically created with entry info. The new pages are listed in reverse chronological order on the homepage (ie, most recent entry at top). So from a functionality perspective, you can think of the site as a user-generated blog, with the form providing all the inputs.

A few other logistical requirements: Each person only gets one submission ideally we'd like to require visitors to log in via Twitter (using OAuth?) to ensure this. However we'd also like the capability to manually provide a second submission to some people. Also, a validation requirement of the form would be that no one can guess a price that has already been submitted.

I'd greatly appreciate input on how you think we can make this work. Are there any existing, out-of-the-box solutions that would suit our needs? Or at least something that's pretty close but will need just a little development work to round out the edges?