Think about what you want to accomplish with the drive and that will probally decide the issue for you. If you plan to rip all your DVD collection there, it would be better to have a NAS since it can be expanded as your library grows. If you just want a place for a few movies (2TB would hold about 300 movies of around 4Gig each) then just get a drive and stand alone device (like the link below) so you can change the drive as needed. Remember the device below is NOT a hot-swap drive (you must have it powered off before removing the drive).
If you want to build a home server I would recommend windows home server. The software costs about as much as an OS disk, but runs an NT build that is solid and reliable (un-like XP or Vista). WHS (windows home server) is currently available in two versions. The link below is for the newer 2011 WHS and the pricing is very good (less then the old V1 cost). The 2011 version had one major change though, they removed the ability to expand storage (a major change). So, if you build a 2011 WHS you will need to find a third party storage solution to run with it. If you find (on ebay perhaps) a V1 version of WHS you won't need to find a third party solution for storage. V1 WHS has the ability to expand storage as far as your hardware can take it. Link to 2011 WHS software ....
Here is an ebay link to a V1 WHS with motherboard (great price for both) .....
Or, if buying a NAS, you would save the time needed to build one. I would recommend something like the link below .....
If you are at all interested in or already skilled in building computers I would strongly recommend building a WHS server. The experience and pride of accomplishment are great and the WHS is a rock solid performer.