Not a local but I was there for the IMSA meeting last year.
Outside parking is free, not that busy, and really close to the entrances. If you've parked at Silverstone or Donington then you've probably walked further to get to the entrance. If you are going for photography then you'll be on your feet all day anyhow, so aren't gaining much by parking infield.
Best spots for photography - turns 5, 6, 7, 8, 9, 10, 11, inside and outside. Lots of options. 6 hours is enough to cover a lot of ground. I did 3 days so plenty of time to explore. Outside of the bus-stop chicane on the back straight is good for some curb-hoping / airborne shots; the downside is it's a long walk and not much else to shoot on the way there.
Grandstands at turns 1, 10 and 11 are free. Turn 1 stand is a bit too far away for decent shots. Turns 10 and 11 stands aren't too bad, but there plenty to shot at ground level.
Mostly there are no fences to worry about. At places where there are fences, you won't be able to get close enough to shoot through them. A few places where it would have been nice to have a small box to stand on just to get a little extra height, but unless you are used to carrying around something like that it's not a big deal.
I used a 100-300 trackside, and something smaller around the paddock. You probably won't need the full 500 that often but I would take it anyhow.
Other things to note, it was hot when I went. Properly 'tarmac melting on access roads' hot. I say that partly because you need to remember to drink lots of fluids, and partly because I nearly ruined a pair of shoes.
My pics from the weekend
https://www.flickr.com/photos/revgea...57709630060676
https://www.flickr.com/photos/revgea...57709629671202
https://www.flickr.com/photos/revgea...57709629694316