I did 1-day M-School in Greer. Awesome experience. Don't worry about weather. Even if it rains, there's plenty of fun to be had. It rained the night before our event, and the track dried up by event time. It rained during lunch/classroom time. The track dried up again before we went back out there. Some events happen on wetted down pavement anyway, so rain really isn't too much of a problem.
Regarding food: I say just eat something simple that's enough to get you going. You'll be tossing your body around those tight corners.....don't want to "decorate" the interior of their M's.
My cousin was out there too and he had to step out of the car a few times because his stomach was---shall we say---unhappy.
I agree about getting as much sleep as you can. It's hard to absorb all the instruction when your eyes are bleary. And there's a lot of [really great and patient] instruction. (When I was on the wetted down skid pad, I had never drifted before, so the concept was totally new to me. I kept spinning the M5 into the grass at first. I did some lovely gardening work for them that day! It was no problem for them. We kept at it, and sure enough, I started to get a decent drift going around the ring.)
As far as being too much of a car nerd-----impossible. Those people live, breathe, eat, sleep, dream BMWs and performance driving/racing. They're happy to talk about anything that has to do with pushing cars to the limit!
Just go out there and have fun. The awesome thing is that no matter how hard you push the cars, they encourage you to push harder!
Cheers.
-dogbone