You are making a smart choice if the mileage is low. I have had excellent luck with 'fleet return' cars that had less than 12K miles, and enjoyed the $5,000 savings.
I would not consider a 'fleet return' that had a bad CarFax, or had high mileage.
I almost bought a CarMax fleet returned 2012 Mustang Convertible that had 9,000 miles and asking price of no-dicker $22,488, but found this lightly used 600 mile example for only $1,400 more.
If you are in doubt of the rental car, then negotiate an extended warranty for free, or very low cost as part of your deal.
The Mustang fleet returns are the best bang for buck out there.
Let us know what you decide!
2012 Base Mustang Convertible
2009 Grand Marquis