Hi Stargazer
Its a great trip depending which route they would use normally done in around 3 days depending on speed and number of drivers and things to see on the way. Registration documents of car, green card from their insurance company some still issue some don't. Licence all parts breath test kit, for France ,warning triangle, spare bulbs, hi viz vest, car fire extinguisher . Photocopy all docs and leave somewhere safe. Only one border crossing that i know of Turkey you buy insurance at the border. In the summer months if intending to use the southern route no need to book the ferries just turn up and travel. A europe sat nav better still a proper map. An easy trip have done it myself 3 times on a motorbike twice in a truck and 3 times in a car.
They have nothing to worry about better driving in europe than the uk.
