Our local airport also flies Allegiant to Sanford. The last two times I rented a car. The last time in particular I really got ripped off on the pricing. The price I booked online got more and more expensive at the kiosk even thought I didn't get the insurance. It was stressful because we wanted to get out of and the foreign lady who was "helping" me was purposely being confusing. I've noticed lots of complaints about renting cars there and pricing. I think ours went from like $170 online to well over $300 by the time I drove off that lot. If you rent be prepared to be very assertive with your agent and watch every screen and piece of paper they have you make choices on. Be prepared for tons of confusing options. My wife was getting luggage. If she had been standing there with me grilling the agent it would have gone differently.
This time we opted to try the shuttle that is bundled with the Allegiant flight. It is way cheaper. We won't have the convenience of driving to the parks or off property but we didn't do that too much last time. The buses work fine for us. It is harder to get around to other resorts if you want to visit but you can bus to a park and from there to a resort.
In general I find getting off the bus is more convenient and closer than parking way far out in the lot.
We also thought the shuttle might be nice on our return trip which will be very early in the morning. We can hopefully catch a few extra zzzs on the way to the airport.
Another thing to keep in mind is several areas (if not all by now) have the type of toll booth where you have to have the tag, they don't accept cash. We got lost last time and wound up having to run through one of those unmanned booths last time because we had no tag. Not only can you get a hefty fine for that, the rental car company might also charge you a lot for that in the fine print.
If you do get the bundled shuttle make sure you call them like 72-48 hours in advance of each leg of your trip to setup the pickup/dropoff.
Have a great trip!