Any new server will be high enough spec for 50 mail box's, id stuck 4gb of ram in... do some research into how much mail store users will need then x it by 50, do a report and get it agree'd by the md or who ever - that way when it runs out of space it aint your fault! - I like to have more than 2x the space I actually need free so I can repair the database's when... no if they screw up!
If you have budget it might be worth considering 2 servers as they are so cheap... I work for a big setup and our clusters are always causing problems so many times we are screwing about wishing we had another server to move the mail box's to.. - thats if they bothered with a cluster.. funny you would think the MoD would have a spare server...
