I like your solution. Seems to fit your requirements and budget.
I would make one mod….no need to drop your exchange prior to the backup. Simply devise yourself an NTBackup job and schedule it.
If I read your post correctly, you have about 10GB of mail. NTBackup will run an 10 GB online of Exchange, to include the all important system state, in about 1.5 hours or less, in real time. No need to drop the system.
Run it at like 1700, by the time Veritas kicks in (figuring a 2100 Veritas run) the bkf from the NTBackup will be waiting, users will never notice, and the system won’t be constantly dismounting and mounting databases, which in my experience, is not the best practice.