What's your opinion on this possible story idea? Has it been used before, should I use it, etc?

Excellent story idea. Yes, the basic idea has been used; but, I'm looking forward to meeting the Corruption Club in print. Get books on police procedure, assassination attempts and successes, FBI stories, undercover cop stories to study the background. Make a list of your characters and write about each one--who they are, how they talk, what they wear--make them real and use the list as you write your story. You wrote out a good plot. Now make an outline or list of actions that will take place. Sit in front of your computer and write, write, write. Then rewrite, rewrite, rewrite. Then, bring it to an agent who will bring it to probable editors. One editor will ruthlessly ask you to rewrite some passages. If you want to go through all that, you deserve to be the best seller the idea might make this story! Good luck.