Currently, we're using resources recycled from APB and Renegade Beta. We're also attempting to contact the developers of The Brotherhood to see if they have anything we can use, though we've yet to get a definitive response.
The SAM Site will be pop-up, as it was in TD, and just like the laser turrets in TSR.
We're definitely looking into adding them in post-release.
It's entirely possible. The Temple of Nod, in particular, is going to have it's own map dedicated to it (It's too big to balance as a main structure, and having it small enough to be balance wouldn't do it justice.)
That's still undecided, however the current plan is for them to not have interiors.