SBS 2003 is a very good OS. Use the wizards and I recommend that you DO NOT use dcpromo. SBS is a special OS and the wizards will install all the security and other required setting to run ‘normally not recommended’ services and process on a domain controller.
You need to consider client license requirements when you go to a domain enviroment. You receive 5 CAL with SBS 2003, so you would need to purchase 15 additional. If you are a 501 3c non-profit, then you should look at www.techsoup.org for assistance with software purchases.
A work group vs a domain is the question you have to resolve. How do you currenty control logons? If each individual has a user name and password, then I would not use a work group. Standardization and security enhancements of a domain are well worth the effort, especially in SBS 2003 since it was created for business with no formal IT personnel.
Let us know more reguarding you infastructur, they workstations OS and if the printers are networked (contain their own NIC) or will be local to a computer and then shared.