Any car is risky.
MOST IMPORTANT - how is it on a decent drive? Notice any shuddering or noises?
Rental-car CON: it's a hack that is treated various ways by many people (potentially, altho yours can't be more than an honest year old). They don't have to "mistreat" it, just that different habits can contribute to many different problems.
Rental-car PRO: often well-maintained by the rental company. And most people w/bad habits or arrogant it-aint-mine attitudes don't get enough time to ruin the car.
The best car I ever "owned" so far (barring my parents' old Lincoln bought new) was a Cavalier - exact same situation as yours. It was 1.5 real years old and had 18k on it, and was acquired from a rental co. by the dealer.
I drove the thing and it seemed like a SOLID (that's the key thing to feel) drive. I kept looking, but sure enough I went back to it.
I had the car for 8 years before finally giving it up for a Monte Carlo. It was great; dependable and solid and all. I loved that little car and felt I was betraying it - but it had some 100k on it or so and had just busted a head gasket, so when I got the bug for a Monte it seemed the right time to let it go.
You can never tell - the best you can do is give it a good test drive.
I took it for a test drive. Road very smooth. Nice, solid ride. Handled well on corners... a lot better than the '00 Mountaineer I'm trading in. I checked it over for evidence of repairs and found no damage anywhere other than a couple of scratches that can be buffed out and a small ding that the Dent Wizzard will fix (at dealers cost).
I checked the oil and it was clean and didn't smell burnt. Same with Tranny fluid. I know that they can swap out fluids, but I checked anyway. It seems pretty solid. Given its a 7 seat SUV, it probably rented out at $100+ a day, so the chance that it was used as a race car by some kid or young adult who likes to beat the crap out of rental cars is somewhat mitigated.
There are no shimmies or shakes when driving. The doors all close with an appropriate "thud". My Mountaineer was close to totalled after a bad encounter with black ice. The entire front clip and both front doors had to be replaced. When I close the doors, they don't sound the same. Its like there's a hollow sound, a weak sound when the doors close, not the solid "thud" that used to be the case.
The Mountaineer is going bad on me. The tranny is slipping and now there is a puddle of tranny fluid showing up under the car. I just sunk $700 into to replace the front link pins and replace the back brakes and rotors (the car has the combo disk/drum rear brakes with the emergency brake in the rear drum.) The pads crack and break up and destroy the drums requiring replacement at $100 each. This was the third time I had to replace the rear rotors because of the e-break pads (not covered under the warranty) So its time to dump and run with the Mountaineer. I wanted to do it before the odometer rolled over 100k, so it was now or never.
This was the best deal I found. I looked at new 05s, but the interest rate was HIGHER! than the used car rate offered by GMAC, and even with the $5k rebates GM is giving, the price is still too high (high 20s compared to $23k for this one) Plus they are giving me $3200 for my Mountainer, which is pretty good considering the milage and everything that I *KNOW* is wrong with it.