Sounds like the successor will be a woman from the wording in the letter. First Mr. Buffett uses "he" and mentions that it's used to avoid strange sentences. When describing the ideal candidate it switches to "he or she" and and when talking about the fact that they have a successor it switches to "person".

That pattern is pretty typical when talking about women but not wanting to give it away from my experience of playing online warewolf (so yeah huge grain of salt needed there)

Edit: Seems like Mr. Munger mentions Mr. Jain and Mr. Abel, both male. I think I'll stick with my initial impression though :P