When you go to collect your car you will need to make sure you take the paper part of your license with you. As most people now have the credit card sized ones is easy to think that this will do - it will not.
If you book with dollar its worth trying to enroll with Dollar Express, as this has it's own express line that really speeds things up when arriving to pick up your car.
As it is your first time driving over there it might also be worth looking into hiring a sat nav. It will take away the extra burden of having to worry about where you are going, allowing you to concentrate on the driving.
On the whole driving in Florida isn't any harder than driving back in the UK. In fact they have some pretty good rules - such as you can turn right at some traffic lights, even if the light is on red.