First time to Italy, Florence and Vencie, suggestions - Fine art, train travel car rentals etc.

Rick Steves also has smaller guidebooks to Florence and Venice -- probably available at your local public library. And downloadable self guided walks on this website.

As an art history expert, you probably already know a ton about the art at the Uffizi, but I did a tour with Context Travel through the museum which was really helpful as I know just enough about art to know that I don't know enough to understand a lot of pieces without some additional information.

They also have a ton of other tours as well, but there are loads of companies that offer walking tours, foodie tours, art tours, etc. I have used that company, though, and I felt it was worth the cost.

I found these books (in addition to the Blue Guides) very useful for background, and for planning what to see in Florence and Rome:

https://smile.amazon.com/Art-Lovers-Guide-Florence/dp/0875806805/ref=sr_1_1?keywords=art+in+florence&qid=1552309802&s=books&sr=1-1

https://smile.amazon.com/Rome-Love-Spelled-Backward-Roma/dp/0875802370/ref=sr_1_fkmrnull_1?keywords=rome+spelled+backwards+is+amor&qid=1552309914&s=books&sr=1-1-fkmrnull

Also, we wound up in Venice during the last Biennale, which was great but a bit much without the advance planning that I failed to do. So you might check out this year's exhibits before going. The Accademia was fantastic BTW, not at all crowded, even in early June.

I read somewhere recently that the rarely visited passage way over the Ponte Vecchio from the Uffuzi would be open more frequently - but, of course the name and dates escape me. I am sure someone will chime in on the name at least.
This year's Biennale is art, next year's architecture.
For train travel, the Man in Seat 61 has tons of details for European train travel - seat61.com
For just these 2 cities, a car will only be a hindrance. Many Italian cities have 'no drive' zones with cameras so you may find yourself with parking and driving tickets when you get home. Just rely on the excellent train system for any city to city transport. If you add in an agriturismo or Tuscany wine tasting or road trip - then that is a different matter.
You will love Italy for fine art. Every church has something to take your breath away. Seeing art 'in situ' (where it was intended to hang) just adds another layer.
Give yourself as much time as you can in each city, however, there are some amazing other items for consideration. The byzantine mosaics in Ravenna are amazing. Should do an overnight there. It is not on the trainline between the 2 cities. Padua, however, is and has the Scrovegni Chapel. It requires pre booking as the fragile Giotto frescos need to be preserved.
Early Fall - bring mosquito repellent. European countries have strict bylaws of when air conditioning can be run and if it is turned off, you might want to open windows, and they don't all have screens. Mosquitos will still be prevalent - particularly in Venice - well into Fall.

I'll second Ravenna. The mosaics were beyond amazing. I actually had a touch of that "art disease"--disorientation, catatonia--that people talk about in connection with Florence when I walked into San Vitale.

These guides did a superb job for our RS tour group, Some said it was like a college-level art history course. You don't need one of those, but you might find them helpful also.

The RS guide has lots of practical information about getting to and from places, including the advice not to fly out of Venice in the morning because it can take awhile to reach the airport (flying in works fine). You won't want a car in either Venice or Florence, though one would help in visiting nearby areas. I'd strongly recommend a day trip from Venice to Padua (easy by train) for Giotto's frescoes in the Scrovegni Chapel, also the Basilica of St. Anthony and the Palazzo della Ragione. Here is the Scrovegni website for making reservations. Allow plenty of time to get there from Venice and, if possible, visit the museum.

A multi-day vaporetto pass opens all of Venice to you and lets you take boats when your feet get tired. Without it, the vaporetto is so expensive you'll use it only when you have to.

While the Vasari Corridor has historical significance, and some beautiful art, it's not scheduled to reopen until 2021 at the earliest.

Grazie, to all for the your very helpful ideas and suggestions; I certainly have a wonderful array of research ahead of me in helping to plan our first trip to Italy.

Again, Thank you

If you are only going to big cities, with no plans for off the beaten path. Absolutely travel by train. Inexpensive High speed trains can be purchased up to three months in advance once you've nailed down your schedule.