Model him in a T Pose with arms outstretched and palms down and you make the job of rigging him much easier. It’s also the default start and end pose for mocap. Posing him after will help bring the model to life. You also might want to google “polyflow”.
Arms at 90deg and 45deg are both popular, I was taught to model them at 90 but I suspect doing them at 45 might help solve problems with the shoulder deforming when animated. Here’s a couple pics from the web (not mine):


The mouth is up to you. Do you want to finish quick or show what you are capable of doing? Modelling teeth, tongue and mouthbag are all fairly standard practices and worth learning.
Scott Spencer has a couple books on character creation which really helped me get started in 3d, highly recommended!