Upgrading from small business Wi-Fi network access points with individual SSIDs
Nothing fancy, just use the same SSID and ensure they are all on the same subnet.
Ensure you use AES, not TKIP. Whether or not you can use an existing server depends on how heavily loaded the existing server is and how much load you'd be adding. A quick bit of searching suggests that Windows server comes with a RADIUS server option (Internet Authentication Service) that can be integrated with Active Directory. That might be your best option if you're already using AD.