It's recommended to use the latest Windows Server OS. As all the intensive work is done by the DMA agents, there are no high hardware requirements for a web server running the Dashboard Gateway. Instead of using a single high-spec server, it might be more interesting to use smaller (virtual) web servers (each connected to a different DMA agent) behind a load balancer, this way the load is also more spread over the DMA agents. You could also start with 1 and add more when needed.