You can get WHS2011 as OEM (it's $50 on Amazon).
As for data - I would not rely on Storage Spaces for extending. If you really care for your data, make sure you have backup strategy as well - RAID (or Storage Spaces) are no replacement for a good tested backup
Depending on your needs there are several free alternate nas software solutions which offer both software raid and the expansion capabilities of Storage Spaces. Heck even whs11 has a plugin for it now (not sure if its free though)