Start in Rome, take the ferry to split in Croatia
The coast of France could be interesting to.
From Croatia i think it is best to take a ferry to Venice, you can't take a car into Venice and parking is expensive.
I haven't seen italy yet. This was the first time a took a car to explore this part of Europe. Next time i will go to italy (but first New York, Boston, Quebec, Niagra Falls).
You discovered the pirce fighters like Ryan Air/
www.easyjet.com Flying is cheaper than driving through Europe.
Best is to go before the shows. The best seasons end the first week in september.
You have to find out if the climate is good in several countries. The islands in Greece are abandoned after the first week in september.