Yes, you'll have problems with shadows, esp. if you have the lenshood on. But that may not be a big problem (taking the lenshood off, that is).
But with larger ultra-wides you cannot avoid the shadow, even without lenshood, see here
First photo shows the trick that you can use, when the upper part of you pic is the skies: turn the cam upside down, so the shadow is cast in the sky
With respect to flashes: I'd recommend the strongest speedlight SB800. Because if you need extra light, it's much better to have the extra power to place it at a distance (so the light fall-off is not as pronounced or when you bounce it off the ceiling or another wall.
Also the SB-800 assists you with a perfect slave mode (the SB-600 too), so you can decouple it from your cam and set it up somewhere else in the room and do some crossfire with the built in flash. That imho gives the best lighting
See also here http://www.cameralabs.com/forum/viewtopic.php?t=390
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
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