SBS is definitely the best way to go. Our first server was set up by outside professionals in 1999, NT 4.0 running Exchange 5.5, Veritas, BackOffice Server, etc. The software bill alone was in the thousands.
For waaaay less you can get SBS 2003-R Premium and it comes with Exchange, SQL, and ISA firewall, everything you will need for the above task. I did the server build and migration myself, as a part-time tech and full-time employee.
If you are technically inclined it is not difficult, especially with a new startup. I would recommend a VPN router/firewall to handle the Internet NIC, a gigabit switch and wireless access point(s) on the second NIC, not routers (watered-down WAPs), DHCP should be SBS' job.
If I were to do it all over again I might just buy a prebuilt machine, as most of my problems with setup were hardware-related. I would definitely choose SCSI or SAS over SATA regardless of who did the build, and definitely choose SBS again. Its wizards are very intuitive and management is a snap.
Read about SBS and view some of the resources online, if you don't feel comfortable doing the setup then do not hesitate to contact your local IT shop for guidance. SBS is designed to be user-friendly but it does require some core knowledge.
Most non-tech people have an outside consultant choose the gear and network layout and get everything working, then let the client manage the system.
Best of luck!
Tony