Setting up a mail server for 350+ people

Im doing a case study where I am setting up a mail server in a hospital for 350 managers.

I also have to buy 350 new pc’s (which I suppose I can order with Office 08.)

I presume the easiest thing to do would be to buy the Microsoft Exchange Server and use that as a mail server but what are my other options?

Lotus Domino Server and use Lotus Notes?

Novell Server to use with Groupwise?

All advice would be great