95 Pontiac Grand Prix V6 160,000miles

My son recently had trouble with his pontiac grand pirx light saying he had low coolant, even though he filled it. He was instructed by an autoshop to put water in the coolant resevoir and it ended up freezing over night. This effected the engine and trnsmission and prevented it from working unless he warmed it up. He took it to a local auto shop and they told him the block had cracked and the oil had mixed with antifreeze. They explained he needed a new engine, or to allow them to tear it apart. So I got it out of there and got a second opinion. I heard it could have been anything from the gaskets to the transmission, but no one had ever heard of a block cracking on a grand prix. A car of that mileage is not worth replacing the engine, and I ran it the other day about 20 miles and nothing was coming out of the engine or the exhaust, so now were just reallly confused. The temperature gauge stayed normal and hot air came out of the heaters.

Should I just replace the fluids or start looking for a new car.