And thanks, Theblazeuk. Without having looked at it yet I was wondering whether all the open source material meant it wasn't strictly necessary to have any specific Dredd/SD source book since all the material/background for a scenario can be gleaned from the comics.
The rulebooks I mentioned wouldn't have anything setting specific, they're all about the mechanics of the games. You're supposed to have the Dredd, SD or Slaine books and the players handbook (PHB) for 3.5, but the freely-downloadable one can replace the PHB.
Oh, if you do end up going down the D&D path, the three 'core' books (though many people get by with just the PHB:
PHB: Players HandBook
DMG: Dungeon Master's Guide
MM: Monster Manual
of if you don't want to fork out for those:
SRD: System Reference Document (released under the OGL: open gaming licence).