You'll definitely want to look at a multi-server setup for that. Put your mail/web services on one box and database/LDAP on another. Also, for such a large installation you may even want to look at their commercially supported editions. Last time I checked (admittedly quite a while ago) the pricing wasn't too horrendous and I've heard good things about their support staff. We've always opted to go with the pure open source aka self-supported version but then again we're running installations with fewer than 300 users. I believe our largest installation to date is ~100 users or so.
I would have thought that this was a small install:)
Agree. If you need multi-servers for 300 hundred users something is just designed wrong. Unless you've got 300 intense power users.