Thanks for your replies. I've been through Heathrow, Edinburgh, Glasgow, and Aberdeen, but this is my first experience at INV. From my research, I figured several of the rental car companies were at the airport while several weren't. I'd prefer to be able to pick up and drop the car at the airport so Avis and Europcar will probably be the agencies I use. I'm staying in Nairn on arrival, and with Avis or Europcar, it sound as if I could be out of the airport in a rental by 45-60 minutes after the flight lands.
For departure, I'm likely to be on the 7:05 a.m. flight. I figured on dropping the car off the night before since it appears that the rental agencies don't open until 8:00 a.m. at the airport, but I will investigate a dropoff at the city centre location if the one-way fee is reasonable. It appears that if I'm fully checked in by 6 a.m., I shouldn't have any issues comfortably making my flight. If I take the 12:05 p.m. flight, I'll probably just hold onto the car and drop it when I arrive at the airport for my flight.