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...
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2003 Regal GS - the beater/daily driver - somewhat stock (yeah right). No its not a mustang, at least it's blown.
2001 GT Black on Black 5 speed
MM Strut Tower Brace/Weld-In Subframe Connectors/Koni Springs&Adjustables/Cobra Brakes/Cobra Rack/Tri-Ax/3.73s/SCT Tuner 56k miles