Not sure about advice with a used Tesla at a Florida used car dealer, but check for options as stated as best you can. Tesla's do not have Model Years like other cars and changes are introduced at random times. Also take a test drive before you finalize the purchase and make sure it tracks correctly and that under full acceleration you don't hear or feel anything in the drive train (pretty normal stuff I guess for any used car). I am assuming you have pulled the Carfax and it has a clean title and no accidents. I would just make a quick call to Tesla to make sure they have not seen any issues with the car and it can still be serviced. Not sure they can provide service history, but worth asking.
I would also check the battery health using "quick method" comparing %charge vs. Rated Range. This will give you a rough idea of the battery degradation (should only be a few %, definitely less than 10%). If you can I would also take it to a local Tesla Service Center and have them check the car over and check the battery health while you are there.
Long trips are a different experience in a Tesla. They are 2 1/2 hour driving (hopefully with AP) followed by 20-30 minute breaks for charging. The car will route itself fine, but if you want to plan your own route I would recommend
A Better Routeplanner. This will plan the route with charging and you can set all the limits you want yourself, like arrival charge level, battery sue, etc.
Forgot to add Good Luck! A Performance Model S is an awesome car. I have had some issues with my P100D, but when I drive it I love the car, it is an amazing driving experience.