If you're not going to be the one that's actively administering the network environment, don't make it overly complex.
Windows Server 2008 introduces a whole host of additional features as compared to Server 2003 - it could appear daunting if you've not actively administered Active Directory in the past.
The "problem" (if you can call it that) with setting up Active Directory is that you *really* have to know know it. Setting it up is easy, fixing it when it breaks is the hard part. If you're running a single server, you better invest in a *very* good form of backup, since it becomes a single point of failure. If the server goes down, the business grinds to a halt.
In your case, with the size of company you mentioned, I'd actually probably go with Google Apps for email, instead of trying to install and maintain an Exchange server on the network.
If you have printers with integrated NICs, then setting up full file & print sharing on the server is recommended. You'll need to double-check for driver support for Server 2008 though. Specifically 64-bit drivers if you're going with Server 2008 R2.
What are you going to use IPSec for? VPN? Is it required? Do you *really* need RRAS and IPSEC VPN? Would it be easier for you to just NAT various ports to allow employees to remote desktop to their office PC if necessary?
Once you install Active Directory, you'll also have to install DHCP and DNS on the main server. These will have to Active Directory-integrated network functions. That is, you'll have to migrate both DHCP and DNS off from the router and let the server handle it. That's if you want Active Directory to function properly.
Again, doing all this creates a potential single-point-of-failure. Lots of things to consider even if the only things you're implementing is Active Directory and File & Print services.
Let me know if you have any questions. What you're planning is right up my alley of what I used to do...
P.S. I run Active Directory at home with most of the services listed above, I *know* how big of a pain in the a$$ just maintaining it can become... Not to mention Disaster Recovery scenarios... lol