not sure about sizing as I have dealt with smaller shops, though the smaller head count in one has had more data collected since we started with Retain.   all on one OES box, migrated a few times in its life

What OS are you aiming for?    One thing to avoid is the BTRFS file system at all on the box, its just a pain with too little gain for this type of application.

and why no archive the MS365 data?   the archive function there is rather weak, especially compared to Retain.  and how will you backup the MS365 data?    their backup is more for their site recovery than for mailbox or message recover.

Being new to both Retain and MS365, I'm not sure what will fit are needs.  As usual there are many internet documents that say all you need is Microsoft and others that you need something else for archiving and backups.  All I  know for sure is we are migrating GroupWise into Retain and then using it only for that purpose.  That's why I was thinking of a single server, SLES, using XFS filesystems on separate partitions so that I can expand if needed.  We don't have MySQL or MariaDB expertise.  So if there is much configuration, maintenance, and troubleshooting.  I may put this on a 2019 server.  

At one customer I had to do it in a similar manner you have to do now. However this customer is a lot smaller. As Andy mentioned this customer is still using Retain to archive O365. So a user has access to old and new data with the same account.

I take care of some other customers similar to your size (fortunately they stay with GroupWise). They are using Retain now for years. So the number of users and amount of data will not be problem. Maybe you need even less storage if you are running more than one post office now.

But archiving such a lot of data will take some time. Take of this. And you did not mention any local GroupWise archives.

Retain will take care of your database. So you do not have play around with settings. You do not have to run a separate database server if you do not want to. In most cases default values will work efficiently. 

Thank you, this is reassuring that I can run in a single server environment.  Once we get a feel for Retain and then whatever MS has to offer.  We may decide to use Retain for MS365.

I stood our GroupWise system up in 1999, and we have not had any retention policies.  I have been working with Mike Silva from (GWAVA now Microfocus) since 2005 to get a centralized archive.  We finally got it, but as a means to migrate to MS365. Long story short, yes we have local archives.  I figure I'll need that capability to expand the archive partition.  Anything else I should be aware of?

To import local archives there is a useful tool which can be downloaded here ...

Thanks we'll probably need this tool.