If registered in Greece (thus carrying the Greek flag) you will have to abide to the Greek law, quite obviously. It can be quite frustrating if you don't speak the language, given the immense complexities of local law. Generally according to the type of boat and its equipment
, it can be registered for sailing within 3nm, 6nm, 10nm, or without limitations. I am not aware of any site listing the requirements in English
I would not advise ignoring those regulations
, given that the Greek coast guard can be surprisingly efficient and quite harsh, particulalry in between Greece and Italy, given the flux of illegal immigrants, transported with boats between the two countries. I think you'd better turn to a local broker, it will spare you a lot of trouble.
Finally, you should be aware that registering your boat in Greece automatically brings you in contact with the Greek taxation system. In case you haven't heard, there's a financial crisis over here and boats are heavily, very heavily taxed. Why don't you simply buy the boat in Greece but have it registered in your place of residence.