Almost all rent a cars in Florida are free one ways.
Yes reserve in advance. Some companies now offer a discount for pay in advance like hotels offer. Wouldn't suggest that. Try Kayak to shop car deals.

Use AAA, AARP, or similar for discounts.

And as someone who drives the keys often, may I ask that you pull over if you wanna look at the sights and water! I try and make it in less than three hours!!