Win2003 Server has IAS, Internet Authentication Service (Radius) but for 250+ users i think you will need the Enterprise Edition. Search the Microsoft web site for IAS for more info.802.11i (WPAv2) with 802.1x and using EAP protocol and a strong authentication method such as PEAP (server side certificate and username,password) or EAP-TLS (server and client certificates) or Funks EAP-TTLS along with Radius is the latest security for wireless. Or some people prefer to still use VPN.
You will need to make some decisions on the type of Access point, Smart or Thin. Smart access points usually have all the config info within and are typically managed individually. Thin are basically a radios connected to a switch, and everything is managed from the switch. the type you choose may depend on how many APs you expect to have (manageability)
It would be very hard to give any real world coverage without doing a site survey or the building. Different buildings have various RF propagation characteristics. Other things like bandwidth requirements of the users and rather roaming is necessary throughout the building come into play.
My suggestion would be to visit some sites like cisco, symbol, proxim etc and read the success stories, white papers etc and arm yourself with knowledge.