What to look for... hmm... avoid the tendancy to get a classical guitar with very low action. Remember, low action is an enemy of the classical guitarist! If you're into flamenco and all that cool rasguedo (?) stuff then you might think about getting a protective clear covering over the body near the sound whole so that your nails don't damage it. Just like any guitar, pay attention to the kind of wood that goes into it, the quality of the tuning machines, etc.
As for serious defects, well I wouldn't by any guitar with any serious defects!