Few Nikon answers here, heee :-)
Assuming that the D70 behaves in a way similar to that of the D100, here is some feedback based on a 1.5 years exprerience shooting landscape with the camera:
- I have been using the sigma 17-35 v2.8-4, basically as an imterim lens, waiting for NIkon/Kodak/Fuji announcements to decide whether I would stay with Nikon or not. Since I mostly shoot at f11, the lens performs great, with only a slight distorsion problem. Flare is also not that great.
Otherwise, the lens stepped down is sharp enough. I guess that the 17-35 from Nikon would perform better but probably by a very small margin at F11.
The 12-24 is tempting, but it seems that it also needs to be stepped down to at least f8 to avoid light fall off at 12 mm.
- In terms of print size, I rarely go above A4 size but others D100 expert users (Thom Hogan for instance) claim that the D100 image quality is enough to print as large as A3 or A3+. I guess that it also depends on the type of landscape photography you do.
Personnally, I feel a slight lack of sharpeness when printing that large scenics containing a lot of fine details (like trees etc...), but the prints can nonetheless be used. At A4, you can print at 300 DPI, you have to go down to around 200 to print A3. I usually prefer avoiding interpolating since the Epson 2200 driver does a good job.
Besides, I feel that the use of a good tripod is mandatory if you intend to print larger than A4. At A4, I feel that the difference between hand held shots and trip shots can not be seen (I am of course speaking of day light shots where 1/500 or faster can used at 200 ISO).
- I haven't done stiching much, but it should work with the D70 just fine.