An easier and more secure system would be to use a dedicated Wireless AP/Router/NAS i.e. a one box solution such as the AirStation™ Nfiniti™ Wireless-N High Power Router & Access Point WZR-HP-G300NH, which would cost around $80.

http://www.buffalotech.com/products/wire...-wzr-hp-g300nh/

All your content can then be stored on an SDHC (8,16 or 32GB) card using a USB SDHC adapter. Power requirements would be much much lower than a PC/Apple Airport extreme combination as the off grid power supply costs will be much higher (Battery/charge controller/Inverter/Solar PV etc)