i would ask him the last time the plugs were changed. If he doesn't know or says they have never been changed, there is a good chance you'll never be able to get them out.
I myself don't mind high mileage trucks, even fleet vehicles. Usually if the truck has high miles it means they were running long distances (far from home) which means if the truck had problems the owner was out a lot of time and money. Sometimes I think a good high mileage fleet truck will outlast a lot of privately owned trucks that only run for 10 miles at a time.
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