A great event, and for a first time visitor probably worth reserving a seat in the 'tribunes' then you can go wander around the circuit during a race, and then go back to your seat.
There is a ticket you can buy on the day for a paddock visit, well worth it just to see all the Bugattis.
The saturday rallye is good to spectate from a cafe or bar, but to take part is v expensive.
I've used the ferries from Portsmouth to France, and car clearance is fine, but not used the Cherbourg route so not sure about that one.
This self catering gite with private/covered parking is about 45mins south of Angouleme, with space for 6 and a pool if you're spending time around there...
https://chezdurandeau.wixsite.com/gite