Dinovo - welcome to Flyertalk and our National forum. I believe that the procedure for you would be to stop at the counter for National to check your driver's license, credit card, etc. Once checked in they'll give you a contract and tell you to go to the Luxury area and pick a car. Finally, once you've picked the car, you'll drive to the exit booth and hand them the contract and the hangtag that gives the information on the particular car that you choose. I'm pretty sure this is how it will work for you - just the added step of having to stop at the rental counter since you are not Emerald Club.
Selection wise, most likely the Cadillac DTS but could be some others depending on what is around at the time you arrive - perhaps someone else will be able to help you better with that information.