We have rented lots of cars and pretty much just shop by price and have never had a problem. Just make sure you stick with one of the big companies and you should do fine.
We have used Expedia and Travelocity, but I'm sure that any travel site would work. Just follow the directions for the rental car selection and it will give you all the cars that are available at what price. The price that you lock into at the site is always the one that is honored at the car rental place (I was worried about a bait and switch). We have also gotten upgraded for free several times as the car we reserved wasn't available. That is fun to get to drive a luxury car when you're not used to one!
Most cars that you pick up at airports are unlimited mileage but it doesn't hurt to check. The only one I've found NOT to be unlimited was Enterprise and that was for a local rental not associated with the airport.
Good luck!
Jayne