Hi Archie,
Here are the answers:
1. Agents purchased for 9.1 are not compatible with 10d.
2.Backup Exec 10d does not support Windows Sever 2008/2008 R2, however, it does support windows 2000 and windows 2003 as well.
3.As it doesn't support Windows server 2008/2008 R2, backup Exec 10d cannot back them up.
4.You need to purchase diffferent set of license keys for Backup Exec 10d, to check for any upgrade path, call Symantec Licensing department.
5.Not pretty sure on this, but Backup Exec 12.5 should support this. Please verify with the Software Compatibility LIst (SCL) for Backup Exec 12.5
Cheers,
Vaibhav29