$150 RT from Detroit to MCO sounds like a great fare. I'm paying $170 (before taxes and surcharges) for Philly to MCO next summer.
As far as rental cars go, Mousesavers.com will have some great tips on how to combine contract numbers and coupon codes to get a great deal.
Keep in mind that the following car rental companies are right in the airport:
Car Rental Companies at Orlando International Airport
On Airport - Level 1 (A & B sides)
Alamo, Avis, Budget, Dollar, L & M, National
Any other companies will require that you take a shuttle bus from the airport to their remote location to do your paperwork and get your car. They can be cheaper but it can eat up a lot of time
Make sure that the price that is quoted to you includes all taxes and surcharges. There are a bundle of them and they can really add up! The car companies that are right in the airport have additional fees tacked on just for being in the airport. Sometimes, the convenience of getting your car right there is worth the extra cost.
Do not take the added insurance if your own car insurance covers you when driving a rental. If renting from L&M, take a copy of your insurance policy as proof. They won't let you drop their insurance without it.
Remember to check for unlimited mileage and the companies policy for returning the car without a full tank of gas. The price they charge for extra miles and refilling the car may shock you!
Find the best price that you can right now and book it. Then, keep checking back for more rate codes and discounts. The great thing about car rentals is that you can make a reservation and cancel it without a penalty. And there is no need to place a deposit when you make the reservation...just the number of the credit card you will be using for the rental.
Oh, and by the way, you will need a credit card to rent the car. Most will accept final payment using a debit card but the initial rental must be with a credit card.