The bra will probably help save the crack from gettin any bigger for now, but what about the long term. I'm sure the dealer should be willing to sort out some agreement bout having the fender repaired or even replaced before you buy the car as it's not everyday that someone actually comes along that is interested in getting a Aston Martin. They'll probably get a nice profit either way...

If you do go for the car better have it repaired properly beforehand cause it will just get bigger as you said, clear bra or not...