its probably better to get a moodle partner involved in the project as you can get specialist advice on this.
But things you may consider - get the hosting of the website done by a web hosting provider( like rackspace or even a moodle partner) to help deal with expansion of disk space in a cloud infrastructure
hardware is a big factor - load balancer .
lots of RAM and virtual memory to deal with simultaneous users. Have a look at companies providing IAAS (Infrastructure as a service) they host everything on the cloud hence saving maintenance cost of the website while giving room for more users.