I think you should start with the places you can fly in and work from there. Since you would like to cruise both the Aegen and Ionian Seas I would say Aegina would be a good spot. Easy access to Athens and placed between both the Aegen and Ionian islands though you would have to go the canal for the Ionian islands.
Ithaca has a well protected harbour and I recall seeing a few boats on the hard. At the other end Rhodes and Kos have a marinas.
If going through the canal Itea has a fairly new marina and you can catch a bus to Delphi
Hope that helps.
Thanks for your reply. I've got the feeling that we'll have to have a couple of trips out there to get to know the ropes. My experience is that once in a new place the 'old timers' will be more than happy to give us the benefit of their experience regarding the best way of getting to where the boats berthed and which anchorages/harbours/marinas to aim for . Thanks again.