Christchurch Wireless AP

I'm planning on setting up an unencrypted Wireless Access Point in Christchurch sharing an internet connection throttled to 512k.

My idea of an Antenna was to mount a USB wifi adaptor (with USB signal boosting adaptor) on a structure on the the roof and add an earth wire just incase of any lightning storms.

Any better ideas? Please also keep in mind that Im trying to keep to a budget.

(I dont plan to give the location of the AP untill project is complete.)