1. Win2K will support software mirroring, but only if the disks are converted to Dynamic Disks in the Disk Management Tool. The trick with Dynamic Disks is that once converted, fault tolerant options (such as mirroring) are not available with partitions that were created prior to the disk being converted (such as a boot or system partition). If you want to mirror entire drives (vs. specific partitions), you would be better served with a hardware solution.
2. It looks like you are going down the right path for your domain organization. I'm assuming the servers at each site will be domain controllers. Realize that with DHCP, a client will accept a IP address from the first DHCP server to answer (which will probably be the local DHCPserver. If the DHCP servers have overlapping scopes, you run the possibility of 2 different servers issuing the same IP address to different computers. Therefore you need to split the scopes. Also to provide true fault tolerance, each scope will have to be large enough to provide IP address to all computers. Also be aware that in order to pass DHCP requests from one subnet to another, either you routers must be RFC-1542 compliant (most new ones are) or yu need to configure your servers as DHCP relay Agents.
3. Reference licensing: Yes you will need a separate server license for each server. How many CAL you need is determined by the licensing mode you use. In the Per Server mode, you would need a number of CALS equal to the number of servers installed X the number of clients likely to access each server. In your scenario, for fault tolerance purposes, every client has the possibility of accessing every server at any given time. Therefore you would need a total of 250 licenses (5 servers X 50 clients) (if all 50 computers are logged on at the same time).
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