I would suggest going to your local computer store (Bestbuy, compusa, etc) and buying a hardware firewall. Dlink, Linksys, and others make some that generally include a 10/100/1000 switch (for the hardware/wired network) and a wireleass access point (for the wireless).
This should get you a long way toward your solution you’ve requested and accomplishing all of the things you’ve asked.
A wireless router and possibly a switch should handle the hardware side depending on how many wired clients.
For file and print sharing, check <a href=”http://downloads.techrepublic.com.com/download.aspx?&q=connect+microsoft+windows+systems&docid=254383″>here</a> for allowing windows to connect to your mac and <a href=”http://downloads.techrepublic.com.com/download.aspx?&q=Macintosh+OS+x+systems+to+windows+networks&docid=264548″>here</a> for connecting mac to your windows network. let the router and individual machine firewalls and AV control the security. Some higher end routers have excellent firewall capabilities.