[url=http://www.roleplayerguild.com/topics/4959-formatting-interest-checks-and-the-first-ooc-post/ooc]This is a good article on the subject.[/url]

A decent inventory for a standard OoC:

[list][*] Title
[*] Details about the plot
[*] Details about the setting
[*] Game rules (what you expect to see as GM)
[*] Character sheet (for players to fill in with details about their character[/list]

Maybe have a look at some OoC's in the section you're planning your game in to see what they do.

I don't think it's necessarily a taboo to use your game as a prologue story for making an actual game, but if you're planning on using anything that your players contribute, you should at the minimum double-check that your players know this and accept this, especially if you might ever profit from whatever you create in the end. You may also want to check the law on that subject.