Do we have any confirmed items that we can buy for them such as a.... Forge maybe?

considering that the 'houses' are large forts/castles, i think its safe to assume a forge will be an upgradable feature

the vampires are magic focused and the dawnguard are crafting focus (says todd)

It says that both houses can be upgraded with special benifts to either side.
i suspect dawnguard will have a werebear super cave of werebearness.

Just saying, and kinda off topic, but they should also offer an option to do a quest or just pay for the reconstruction of the

Spoiler

destroyed places in Solitude and Whiterun