Like sexytw mentions unRAID isn't suitable if you want to run XBMC, as unRAID is a dedicated operating system and you can only run unRAID, if you wanted to use another computer with XBMC then you could use unRAID on this one and access the files over the network.
In terms of which HD you need to purchase, it's pretty much up to you in terms of size & cost, and the number of HDs is dependant on the type of RAID you want to run..
The Duo I believe supports RAID 1 which can mirror two drives, which is pretty much the simplest form of RAID along with RAID 0 (RAID 0 is a bad idea and joins two drives)
The next option up is RAID 5 which provides a parity drive, which is slightly better in terms of protection, but involves 3 drives, I suggest a bit of googling here, otherwise this could become a very long post.
Personally I'm running RAID 1, and some others on here are running RAID 5, if you have any specific questions, were more than happy to answer
EDIT: And welcome to the forums BTW!!