Engine needs rebuild or only head gaskets?

My son has a 1999 Ford Taurus wagon, 3.0 liter OHV regular V-6. 102,000 miles. A few weeks ago he overheated it due to low coolant due to a slow heater core leak. He was on an interstate and claims he stopped as soon as it overheated. Who knows!

It was trucked home. Next day he added coolant. It ran fine. He took it to a chain "tire" gagrage for repair and they pulled a spark plug and said it had water in cylinder and therefore needed head gaskets at $600 or more. Time was too short and he had to return to college. Some water vapor was becoming visible, but it ran fine, aso he drove 200 miles, but on the way the steam exhausted was getting worse and he had to add a gallon of mixed coolant over the 200 miles. The car has sat since. I went there and checked the oil and it is the color of coffee with cream. An engine salesman told me the engine is ruined after driving 1/2 mile with that kind of diluted oil, and it should be rebuilt rather than just getting the head gaskets or heads repaired.

Question: If we only fix the heads, what are the odds that within a few hundred or few thousand miles the engine will need a rebuild?

Question: If that cannot be known with above info, is there a way to inspect engine to determine condition of engine, otherthan taking it out and tearing it apart?

The body and interior are in great condition, and it has had many recent new parts added, including rack and pinion. Price quotes for a rebuilt engine are $1600[Rino] and $2200[Jasper, plus $1500 or more labor. Considering the heater core leak still needs repair[est. $600], it seems not worthwhile to rebuild engine when I see used wagons for $2000 or less with 100,000 miles or less. But if new head gaskets will do, it seems worth it. But the worst case would be to repair heads and heater core and then have engine fail!

Help is needed soon because college is done in 10 days! Thanks for any ideas!

Bill