There are a handful of shots to get:
1. A head+shoulder portrait of him on-stage.
2. A wide shot from the back of the hall showing the stage, speaker, and audience.
3. A wide shot from the side of the stage showing a profile of the speaker and the audience.
4. A full-body shot of the speaker on-stage.
5. A waist-up shot of him and whoever introduces him, should the opportunity arise, during a handshake or other formal on-stage greeting.
Basically, the 50mm isn't gonna be much use to you. You need to borrow or rent a 200mm lens, at least, and you'll probably wish you had something wider. f/4 will probably work fine, since most speakers are fairly well-let.
Any photo that doesn't tell a story is a waste of time. You're entering the world of photojournalism and, without a story, it's just a photo. You need faces, you need emotion, and -- when possible -- you need (inter)action.
Expect to shoot a hundred (or a few hundred, even) photos to get that one that's good. If you take just the 50/1.4, you might be surprised to find that none of them are printable.
When I was first learning photojournalism techniques, I read a few books on the topic. One author said, in his first paragraph, "If you learn nothing else from this book, learn to set your aperture to f/8 and learn to be in the right place at the right time. Don't worry about technical perfection. Focus on telling the story."