I would strongly recommend that you get a historian on board as a second supervisor, as Pink_numbers suggested. An ancient historian might be best of all, but failing that any historian would be better than nothing :p

I agree that you need a historian on board too - they will guide you through the humanities side of your degree. I'd also agree that that is epic, I'd say (and stand to be corrected) that its way too big, any historian would tell you that. I think your suggestion of keeping it down to a specific area is far more appropriate - I mean, you're talking a massive span of time, and a vast area - its just not possible. You could even keep it to one area - just the Med would be a huge undertaking. You have to remember that 80K words really isn't very many... it sounds a lot, but trust me, you have to be focused to get a study into that - a worldwide study would be a doorstop of a book - heck to do it right you'd need several volumes!

As far as your concerns re standing on the shoulders of others - that is what we all do - we take what others have done, examine it, contrast and compare it with the work of other researchers, then consider it, use our sources (I've used many not used before) and then make something further of it. We have to make a contribution to knowledge - by collecting together the various threads from other writers and then offering some new insight thus furthering the course of research you will have done that. You don't have to be saving the world, finding a cure for cancer (although that would be good) or creating a new species to be worthy of a PhD - the PhD can be seen as an apprenticeship where you learn the advanced research techniques that you then use in your career. Don't put it on too high a pedestal ;-)