Haven't done Goodwood, but have done LM. There are a ton of very good logistics guides on the LM sub-forum on Ten-Tenths.com. It is an even that benefits from a lot of advance research. There are differences in admission and paying for certain grandstands. You may want to buy tickets for multiple grandstands. There are tricks like joining the ACO which can get you access to some on-site hospitality. Yes, you can sleep in the car. We got a hotel room 20 minutes away, but napped in the car during the race. The span of darkness is short given the time of the year, but don't miss the night time. Some of the most magic moments of my memory were walking around at 3am between the pit exit and Tetre Rouge.
Don't expect to get anywhere near the pits or the cars without special credentials. There is a Friday pit walk, but it is mobbed. Wed and Thursday are great days to there - smaller crowds, easier to get around. There are shuttles during the race to get around the circuit, but they are overcrowded and it isn't easy. There are also a lot of fences, so finding clear viewing or photo points can be a challenge.
PM me if you have more questions.
