ReadyNAS nv+ vs nv+ v2 vs 104

I would not get an NV+ (v1), if this is indeed the unit you were looking at. It's relatively slow, doesn't support >2TB drives and is unlikely to get a lot of development support from now.
The max storage capacity (with redundancy) with the NV+ (v1) would be about 5.4TB.
Anyway, I suspect you are actually looking at the Ultra 4 (that's what the review is for), this and the Ultra 4 plus and Pro 4 are good machines; they are also now legacy products and may not receive as much development - Netgear has moved to a new OS which is only available on the new NASs like the 104 and 314.
They all, however, are probably fast enough for you, and can take 3 and 4 tb drives - lots of 3 tbs on HCL, only 1 4 tb.

Here is a calculator which will allow you to determine how much capacity you will have with different disks:

http://kossboss.siigna.net/xraid.html

You're considering the availability of addons - you may find over the coming months that the newer models have more available than the "legacy" models.

Have you considered the Readynas 314, it's more expensive but if you want or need a faster processor and you can afford it then that's what I would get.

Oh , please try (initially at least) to post on one forum section and wait for a reply, rather than posting the same questions in multiple areas simultaneously. You will almost certainly get a reply if you wait a while, the forum members are very helpful!