If the motor was overheated you would know it. It would be using coolant, blowing coolant out, and generally running lousy. I never heard of someone preemptively surfacing a head without some evidence of a blown head gasket. And NEVER just one on a motor that has 2 heads. One obvious sign of overheating is the plastic screen in the oil filler. If it is melted or missing then you can suspect the motor has been overheated. You confirm by testing for combustion gasses in the coolant. Pieces of spark plug are concerning but I would try to get them out the spark plug hole before removing the head. If someone had to take 20 thou off the heads they were severely overheated and to try to flatten them and have to take even more off is way too much. I am suspicious of your measurements and would recheck your cylinder height in several places. Whatever you do to one you should do to the other head. If the heads really are cut that much you should not waste any time or money on them. Find a good used set and clean them up. As for honing marks, even on a well used motor you could still see honing marks so that by itself is not a good sign of previous work. More obvious would be sealant, the gaskets, wrench marks on the bolts, spots where things may have been beaten or pried on, that kind of thing.