I run IT in the start up company I work for. Declaration: I have no formal IT training, it's not even the main aspect of my role, but someone's got to do it in a 10 person business. That said, this is the second time I have been responsible for setting up from scratch all the IT for a new company, and it worked very well last time!
The most important thing is what your end users are expecting/needing/used to using. And for me, both times round, this has meant MS Office (PC), with Outlook being core to day in day out working. If I had tried to introduce even Entourage, let alone the suite of Mail, iCal and Address Book, there would be a riot, and a lot of downtime as people get used to this, no matter how intuitive the Apple solution is.
I strongly advise against running your own email. What I do is use a provider (there are many) such as Rackspace (mods - I'm not sure if I'm allowed to mention names, apologies if not) who, as a previous poster above says, will run the backend MS Exchange platform for you. All you need is your domain name to use this with. This will typically be less than $10/user/month and for this you get full shared calendars, contacts, outlook web access, and often a free license to the most recent version of Outlook if you need it for your users.
What I have then done (being a Mac guy) is to use a single Mac (previously a Mac Pro, but now one of the new server Mac minis) as a file server in the office. All the PC users map network drives to this. To them they notice nothing as regards it being a Mac where all there files are! I then get to use a RAID1 array with the twin 500 GB disks in the mini to protect against hardware disk failure, and will couple this with a 1 TB firewire drive to act as a Time Machine backup. Then I have a small portable 500 GB drive that I use once a week as an offsite back up that I clone the server drive to using Carbon Copy Cloner (excellent software). If you also want to automate offsite back up you can use the Apple mobile me/backup sync service, or something like Jungle Disk.
I have used Mac OS Server 10.5 previously, and found it to be utterly overkill for file serving alone. This time I plan just to use standard Snow Leopard install.
The one caveat to all of this is that if the company in question is going to be going through a rapid rate of growth of user numbers, then server software will aid you in mass set up and performance.
Hope this is useful.