Get Fallout 3, its a brillant game, then play the various DLC. Then get Fallout 3: New Vegas.
By the way like you I prefer southpaw settings in FPS type games. Its annoying that some games do not support it so I went as far as getting one of my joypads modded so the sticks are permantly swapped and I have normal controllers for playing other games. I been able to play great games like Dead Space that I would not have been able to play properly had I not modded one of my pads to southpaw. There are some firms that do the mod for you or if you are handy with a solder there are instructions to do it yourself but not adviseable if you are not clued up.