How to choose a Media Server for a 1500 employee non-profit organization?

I am recently looking for a proper server for a 1500 employee company. Basically streaming video and audio for educational purpose would be sufficient – I am not looking at VoIP functionalities. Just wondering what kind of configuration should i look at? How much bandwidth would it occupy during peak access time? And of course the price – could you recommend some places for an estimation? What are my options for security protection? Is there any template for business case? Thank you in advance. R