Are you sure it is a cruise you want? With the kind of touring you want to do you might consider a land tour. Cruises don't give you enough time in port to do any extensive touring--you can see a lot, and private tours are surely the way to get the most of the time you have; but time is limited and, as you say, so are funds and private tours are quite expensive.
And I don't know of any cruise that takes in Venice, the Greek Islands and France (Paris) other than maybe a segment of a World Cruise; but you want to go within the next month, so time is pretty short to plan that kind of adventure.
I think you should sit down with an experienced travel agent who can show you your different options. To me, it looks like you are trying to do too much in a limited time.