It was dry last year, and very dusty in the rally area. The rally section is quite far from the main area, but they have a tractor and trailor shuttle service, though the queues do get quite long. Still probably works out quicker than walking, but it was an exceptionally dusty, and very bumpy ride!! Recommend you have all your camera kit in a bag before you join the tractor queue.
I only took photos within the rally section in 2009, and left my 100-400 lens on, all the shots were taken at 100mm, and the odd photo would have benefitted from a slighter wider lens, so I think your 70-200 will be ideal. If you're also planning on taking photos of the hill climb, you would be better off with the 100-400 - plus the Red Arrows do a good display there.
Friday is quieter, but is still busy, and it's definately an event where it's worth planning in advance so you can spend two days there or even three!