I was searching the net for storage solutions a while back and found this document from the National Australian Archives that details using shipping cointainers for archival storage.

http://www.archivenet.gov.au/ARCHIVES/parbica/Documents/ShippingContainerSpecification.pdf

The document addresses the very important environmental contol factors that would be useful in this application. I purchased a container a year back for about $2500 delivered. If you have the space to put it, its a good option.