Well, it wouldn't have
to be diesel - the petrol v diesel would be a choice based on the availability of fuel at your departure point and at Kastelorizo.
Given that your journey is about 450km = 279miles, then your boat will need to be designed around the fuel tanks, and if you are planning to make the trip regularly, then you may want to have permanent underfloor tanks rather than flexible deck tanks.
This thread may be of interest to you - Fjord Transit
their crossing of the North Sea was, I think, roughly the same distance (or a little more) that you are planning?
Codprawn has suggested three RIB manufacturers who build good boats. Scorpion are manufactured in Greece under license by Ribco and are one of the best all purpose RIBs available - I'm led to belive that the Scorpion (Ribco) has a drier ride than the Cobra, and a softer ride than the Revenger.
However, I think there's much more to decide before narrowing down Manufacturers - fuel type (ie petrol or diesel), number of engines (ie 1 or 2) and style of weather protection (ie cabin or open) being possibly the most important.