one with lower miles could inherently have fewer bugs to work out (IE older one may need other work done..) but no guarantee on that, previous owner of a high mileage one may have been meticulous, not happy if there's even one rusty bolt, etc, where a lower mileage one (not likely in a cobra but..) may have been abused, never taken care of, repaired, etc...
What it comes down to is checking a car out in person, looking it over thoroughly or have a mechanic look it over, and taking it out for a thorough test drive...
the stock block, even with fairly high miles, is pretty sturdy up to a point still, I would say you shouldn't have to upgrade it right off the bat...