To me, the best briefings would be something that represents what a company commander would expect in getting his assignment from the battalion commander. In many respects, the scenario designer is acting like the battalion or regimental commander in handing out the orders and the player is the company commander receiving those orders.
As such, a force overview is appropriate. If reinforcements have been requested and the request granted, then they can be identified (perhaps not in detail since the company commander may not have control over those forces until they enter the battlefield.)
Opposing force knowledge can vary from detailed to none. My reading of personal accounts shows that there was a wide variability to the knowledge of what to expect prior to a battle. In some cases, the intelligence may be outdated and may under-estimate the opposing forces but just as well may over-estimate the opposing forces.
A good book that I am reading now that provides descriptions of how the briefings were provided in the US Army during WWII is "If You Survive" by George Wilson. He was a platoon and then a company commander during the campaign through northwest Europe.
Carl