I ve done one of theses rallies
You might want to
Try make friends with some of the others going
Get cb radios
Buy an estate, (can sleep in back/more room)
Make sure the car has good brakes/vented
Map of ways to go,
Two sat navs
And I remember we have to buy a road tax thingy for a day to pass through a country but can't remember what one. (they cheaper a year on eBay than one the day then sell it on after)
A credit card for fuel (then split the final bill by how many in car at end)
Look/book for fairly good hotels not near the end.
Also will you be driving back in the car?