... okay, this may be hard to comprehend.
Believe it or not, test driving the same model car would be almost as pointless as not test driving the car at all. the only way to make sure its in working mechanical condition is examine it in person, and test drive it. If you go to a dealer, chances are that you will be driving a car that will functions quite well because they have mechanics make sure nothing is wrong with it when you are about to purchase it, because the dealership would gain a bad rep and ultimately suffer in the long run if they were selling lemons (cars that look good but are actually about to break down or "go sour")
so driving a car from a dealership, which is most likely equipped with all of the OEM parts that originally made up the car, would be dramatically different than driving the car that some average joe, with internet access to some import tuner messageboard and a bunch of money to blow on aftermarket parts, rigged up in his garage using daddy's toolset... god knows what new parts in turn fucked up original parts or if everything was installed properly ect ect.