That sounds like a great road trip I've only been to france the once so far in mine, Dover to Calais then a blat to lemans and back for the race.
I had no problem getting on and off the ferries, but becuase my car was so small they found a tiny corner for me to park in on the car deck which was open to the element so be prepared with a cover for the cockpit to keep the water out if you have to. The only time i had ground clearance issues was over the speedbumps at customs, 2 up with a weeks worth of kit and 2 cases of kronenburg strapped to the roll bar does make the back sit lower than usual
Regarding what to take, i carried only a simple tool kit to allow me to do the obvious together with some spare fluids. Make use of the space in the engine bay for this so you can keep the boot box for clothing. I cabled tied everything to the side of the battery on the scuttle. Have a think about where you might get spares if something breaks. With my car being Mazda based i carried a list of all the mazda garages that were along my route. As for spare parts I carried only a throttle cable, cable ties, a length of wire, some scotchlocks, insulation tape and a roll of duct tape.
If you're unfamiliar with your car do you know what different cars and their years which the donor parts are from? Assuming probably not will you have the means to access the boardroom while you're away and post picture to ask us (smart phone etc) so the local garage can then source the bits for you? One always hopes that nothing will go wrong, and rarely does it, but it never hurts to be prepared.
Waterproof bags such as those made by ortlieb are good to ensure all your kit stays dry.
My passenger found that their comfort was improved by riding with a rucksack under their knees.
For breakdown cover i used Britannia Rescue. Check with whichever recovery service you opt for that they will cover kit cars, it has been suggested in the past that not all do.
Most of all have fun and enjoy yourself