Thanks guys. I've just read Whatever happened to the man of tomorrow and loved it. Never thought I'd say that about superman. However I just flipped through my copy of The Killing Joke and realised it really isn't appropriate for my potential audience. I'm thinking Dark Knight Returns as a Batman comic, but I'm open to suggestions.
I've got the above plus Year One, Arkham Asylum, Hush and RIP. Out of those I'd go with DKR or maybe Hush. Any others that I should consider? Remember they need to be quite short self contained stories and not something that will require more than one paperback to cover the story.
Thanks so much guys, appreciate all the help.