What are the distance between the buidlings? If the distance between the buildings is more than 1 mile (a restriction imposed by IEEE), you need to implement a propietary solution. Do the vendor provide such propietary solution?
What kind of encryption is supported? (you may want to use AES for enhanced security)
Do they support WPA? (AES requires WPA v2 for inter-vendor compatibility). If you do not use AES, you may want to use TKIP, which requires WPA v1.
What kind of authentication(s) are supported? You should use a mutual authentication method, for example LEAP.
