Keep in mind that the fuel is always paid by the customer. Some car rental companies will ask you to leave the tank filled at the same level that it was before renting, other companies provide a full tank and will accept it filled, which is the best way.
Most Greeks speak English, but if you are planning to visit some villages, the best way to communicate is to learn some words or expresions in Greek. If you need guidance while driving, you will need to learn how to ask directions for a road, a gas station or a parking! Even if someone cannot speak english they will try to be helpful!
Keep in mind, that if you are planning to visit Thessaloniki with friends, a lower category car, does not have enough space for the luggage of 4 adults!