What kind of business is this for? Will any ecommerce (online purchasing) be conducted with your business? Is this business a home business or in a small office location? Are you wanting to host in-house internet/intranet websites and email?
As celegorm suggests ... hardware firewall would be what you'd need in the firewall dept. Software firewall isn't necessary. Hardware firewall like SonicWall or Cisco PIX (PIX 501 for your situation) would be examples. A benefit is being able to create VPN's to login remotely.
***There are some firewall hardware solutions that include wireless capabilities.*** I haven't had a need or opportunity yet to work with these, but they might be a way to kill 2 birds with one stone. Wireless and hardware firewall all in one. Here's a sample of firewalls: http://www.newegg.com/Product/Produc...TabStoreType=0
A simple 24 port linksys switch like the following probably would be sufficient: http://www.newegg.com/Product/Produc...82E16817201106
You might be able to get away with a 16 port switch, but you're kind of limiting yourself according to your numbers above.
Are you wanting to have a work environment domain utilizing a central file server?