I agree with Preston, rent a SB900, it has more capabilities than the SB800, do this now, because it has a huge learning curve. Also, go to Kelbytraining.com and join for a month or two and watch his video on the "SB900", and "Small Strobes, Big Results" by Joe McNally.

For lenses, I would rent the Nifty Fifty, 50mm f1.8 prime. It is one awesome little lens. Actually for just over $130.00 US, it is one of Nikon's more affordable awesome lenses.

Second would be the 70-200mm f2.8, but this lens weighs 4 POUNDS. It is not a lens to take lightly, but as a zoom it is phenomenal. This lens has wonderful bokeh, and is very silent and great clarity.

You didn't mention how far away you would be from the judges. If you are going to be close, check out Nikon's 24-70mm f2.8, another one of Nikon's great lenses. This one is on my wish list.

Your other option is to rent a whole setup, Camera, flash, and lenses. This will cost a small fortune, but if it gets your foot into the door, it will be worth the cost. If you go this route, rent the Nikon D700. I just purchased mine, and I have gone as low as ISO6400 with NO noise shooting in JPEG! This is one awesome camera. Obviously, shooting in Raw with the questionable lighting you will have is much better.