I can't speak from 1st hand experience, but i have been "pit crew" for RowdyR32 at Sandown, he's pretty blaise about this stuff and went out with stock radiator and no oil cooler. Just did an oil change before hand (2-3K old oil anyways), checked brakes, tyres, etc. (should of paid more attention to the susp. on his GTR before going out but that's another story) and bought some high-quality brake fluid.
As these were super sprints, 6 laps or so long, temps weren't too bad.
But the brake fluid was the problem, as he didn't change it over before hand, so by lap 4-5 his pedal was pretty much on the floor coz he'd boiled the old stuff. So, between each session I had to help him flush the brakes, and top up with the high-quality stuff.
So i know the one thing I'll be doing for certain before hand, if/when i get out there, new brake fluid!
Another thing is, if this is a CAMs event you'll be required to have an extinguisher, but even if it's not I'd still carry one (maybe even two). I've been meaning to get one even though all i do is drive on the road.