Re: Preperation for much better compition
Focus on your strengths. That scenario is no time to try to do things at which you're not strong and confident.
As stated earlier, be prepared for the game to be much more physical than you're accustomed to. Don't let the physical play take your head out of the game.
Move without the ball. In a charity game guys tend to be loose and lazy defensively. Be the guy who presents a good, open target.
Show no fear. Get caught looking "wide eyed" and you're doomed.
Have fun. It sounds like a great opportunity for you to see where you stand as a player.