You're going to want to split that environment for scalability reasons. I've had things get ugly every time I've tried to monitor more than 1200 servers on a single Balance instance, and adding resources didn't help much. Also, make sure you have enough proxies to ensure that no proxy is monitoring more than 600 servers.
It's easiest to run one Balance VA per vCenter server, if the numbers work out.
Your customer may run too many VMs per vCenter to monitor all with a single VA. I've had one customer with over 2,000 VMs on a single vCenter. In that case, Balance allows you to include/exclude at the vCluster or host level. You'll want to use this capability to split the vCenter into manageable subsets.
The disk requirements listed apply to the storage behind the Balance VA.