If you don't need to change tires or anything, 7 am could be a little early. You'll definitely want to get there early enough to check in, get your car in order (remove spare tire and other junk, put numbers on, etc) and walk the course a few times by yourself. Probably 7:45 to 8:00 would be perfectly fine. I usually show up as early as I can, as I would rather have too much time, then be rushed.
Edit - I usually have a duffel bag full of stuff (clothing options for the day, some food, sunscreen, etc). Since it will be raining, you may want to bring a large trash bag to keep things like a duffel bag dry while you are running.