Release of version 11 is coming imminently. It’ll probably be generally available before you’ve had time to procure and configure hardware and software for this project so it would make sense to plan to use version 11 from the get-go. Veeam releases are unlike Microsoft releases in the sense that they are tested prior to release… haha. I can’t think of a time any of the features I regularly use being compromised by a version release.
Anyway, the new version gives you the ability to create an immutable disk based repository on a specially configured Linux server. Disk is actually very similar to tape in terms of cost these days, so why make life hard for yourself with tape.
The main concern when it comes to the 400tb size you have will be restore time in a DR scenario. Day to day backups will be manageable on basic hardware as only incremental backups are taken after the first full backup.
So you must find out from a discussion with the organisation owners what their cost v.s. restore time priorities are. If being able to recover their entire exchange environment very quickly is of the utmost importance then they’ll have to spring for a replica environment which will mirror the performance of their primary production hardware. Anything less than that will see slower, possibly unacceptable performance. In this configuration scenario you would configure replication through Veeam. The failover time is very quick, not instant… but almost instant.
If they would rather spend less and accept the slower restore time they can get something cheaper. You could provide a middle road where you have a copy of the system which can be brought live very quickly and a different backup to restore the databases slowly while the live system allows people to continue working with new emails. This route requires some careful consideration and thorough testing to get it working properly.
The ideal configuration to assist with fast restorations if you cannot do replication is to utilize an off-host proxy server with a repository directly attached local to the Exchange server data. Connect this into your SAN backbone rather than depending on the LAN or WAN. The directly attached disk provides a local copy which can be restored quickly. For full data protection you would have to copy the contents of the local disk to your off site repository.