We'll have to choose what system we are going to base the scenario on. My suggestion would be D&D 4e, cause that is the newest version and got a lot of players, but pathfinder is of course also an interesting option.
We also have to decide on what level we aim at. The obvious would of course be 1st and our scenario taking people to 2nd or higher when finished. However, we could also start off with a higher level to make the challenges more interesting and to make it more credible that the city would use our adventures to help them.