Hi. I'm implementing approach for 7000 students, simultaneous - 3000 users. We lease a cluster - several servers in a data-center. I'm sure you'll have to do the same. The point is how you'll organize the servers and their infrastructure. That's the "know-how" approach. I'm working on a paper about using clusters, nginx, caching, accelerators, load balancing, distribution of tasks, etc. Anyway the system requirements can be in a wide range and they depend mainly on what kind of courses and data you are going to store in Moodle, e.g. using streaming video & audio, web conferencing will highly encrease hardware & network requirements.