Hairline cracks can exist and can be very difficult to detect; even tiny ones can let in a lot of water.
The only truly reliable way to establish whether there are any in a used boat is via an on the water test. Cracks seem to occur most frequently around the drive wells, the scupper holes and in the cam columns of the older 'screw-down' drive locks but as I say: they can be very difficult to spot. If you can't do a good long on the water test to prove that the boat is watertight and you can't get a cast-iron money back guarantee you should walk away.
The other potential area of concern is the drive-well locks: if the boat has screw-down drive locks you should look inside the boat to both sets of cam columns; these are the plastic cylinders inside which the drive-locking bolts run. If any of the cam columns are cracked, walk away.
As long as the hull is watertight, do not get too concerned about scratches on the hull unless there are any really deep scores for which you should inspect the hull - the polyethylene is very resilient and unless is has been attacked by/ dragged across something really, really sharp it is unlikely that scratches will be anything other then superficial. Also, hull scratches can be repaired with plastic welding gear (this is not likely to be successful in the vicinity of the drive wells due to flexing in the plastic due to pedalling action).
You should check that the drives, paddles, rudder, hatches are all present and correct plus any extras that might get thrown in e.g. Cart, PFDs, sail kit, spares, etc.
Don't be afraid to walk away: the newer models do have a number of advantages including dual steering positions, better drive technology and a substantial warranty.
On the other hand if you are prepared to do your due-diligence to establish that this boat still has life left in it and you are willing to accept the risks, you might well score a bargain... I have done so myself