I'm a FOX body guy who doesn't know much about the 03-04 Cobras...been out of the Mustang world for a while....Thought that buying a 03-04 cobra would be better/more reliable then building-up a fox body. (i want to go fast, but also want reliability? is that possible?

I'm currently looking at a '03 cobra Coupe, 25K miles....for $26,400...from a ford dealership. When I test drove the car Iheard/felt a slightrear-end "clunk"when shifting at low RPM's...is this common?
The salesman stated there were no aftermarket parts on the vehicle...when I opened the hood, I saw the K+N FIPK...not the most significant upgrade, but.... To the naked eye there didn't look to be any other mods...exhaust..ect!! The car seems to be in pretty good shape except for the crumbs and stuff...
Would I be better offgetting one through a private sale? Any input appreciated...Thanks