David, thank you for reminding me - you are absolutely right, there is no avgas available at ULLI, but a typical GA plane with a long-range tank option will make EETN-ULLI-EFHF without problems. Speaking of closer places to land, EFLP is a possibility, it is an international airport.
For the visa, you do need an invitation but flight details are not necessary. Even if they ask you, just say you will be driving your own vehicle, which is a very popular option anyway.