1. Anyone know any good campsites anywhere between Igoumenitsa and Athens, and then Athens to Turkish border?
When driving south from Igoumenitsa, the 1st campsite is about 7 kms from the ferry, on your right. Good food.
Inbetween I didn't stop. On the middle finger of Chalkidiki in the south-eastern part of the small town of Sarti there's a nice camp. The one in the north is a bit ramshackle.
2. Is wild camping allowed?
I reckon the Greeks are just about the most laid back people of Europe. If you are on any dirt road and find a secluded spot next to the track, I'm sure nobody will chase you away.
3. Are there any good areas for exploration with a 4x4?
Dunno about 4x4, but the Pindos mts. still offer lots of dirt roads in varying conditions. I explored the small backroads in Chalkidiki on my bike, which was lovely
No traffic and always a great view across the surrounding landscape.
4. How much is diesel?
Don't know the present situation, but in April fuel was about 25 - 30 cents cheaper than in Germany. Our fuel prices are more or less equal to yours
Only when we pause to wonder
do we go beyond the limits of our little lives.