For disks, watch the 4tb externals to save a good bit of cash. They are easily opened to insert into a server or nas. This one
was as low as $140 just a few days ago. It's back up to $172 at the moment. Use a site like camelcamelcamel.com
to set a notification, they'll email you when the price drops to a price point you specify. This way you could get 4 x 4tb for about $600. If you use software raid like flexraid you'll give up one of those as a parity drive leaving you 12 tb. For your size of a collection, and with drive prices where they are, I think it would be a mistake not to go for 4tb drives at this point.
Or, depending on how anxious or not you are to dump cash on this project, maybe just get a couple 4tb drives and keep using your 2tb drives and put them all together. Then add/replace drives in the future as needed for capacity increase.
xbmc isn't a server so it doesn't run on the nas, it's a front end. You will easily be able to point it at any media on your network.
NAS boxes are very expensive, may be better off building your own at your price point. You may have a pc in your home already that could handle the task, might just need a bigger case to house all the drives.