Kate,

You don't need to book train travel now. You can get your tickets once you are there. Usually the day before is more than enough. If traveling on a holiday or weekend, you might want further advance.

Buon viaggio!

As far as a car on the AC is concerned, you will get two very divergent opinions. I always drive on the AC and have never had regrets. I like the freedom of stoppin when and where I want and enjoy the challenge of the narrow winding roads. Traffic can be difficult depending on the season. I've driven the AC in early May in both directions without problems. Do you drive a manual shift? There is usually a pretty steep premium for an automatic.

In Venice, the Locanda Fiorita is very nice and is well located at Campo San Stefano which is an easy walk to San Marco. It is in or close to your price range.