I dont know when the US phased out leaded petrol on new engines, but it was 1986 in Australia. The fact that they've called out unleaded compatible pistons suggests it was a leaded engine, and if the US phase-out date was the same as Australia, that means your engine is at least 25 years old. Not that this is necessarily a problem, just something else to think about.
I agree with boxhead that paperwork is absolutey key in such a purchase, however for a Cobra that is 15 years old and not from a top 1 or 2 "volume" manufacturer I would be having it thoroughly looked at by your local Cobra experts (read: current manufacturers or other locals with wide-ranging Cobra experience) , and if you think the car has serious potential I would maybe even go so far as to fly in one of the well known Australian Cobra mechanical experts for a day - a days labor and $200 in airfares is small price to pay for real piece if mind. Or just ask on here
