1999 Mercury Sable Engine is heating and water is leaking f

I drive Mercury Sable, 1999, and it has about 62000 miles on it. When I use A/C for long time, water has been leaking from the coolant but only when it is parked. I thought it was because of the long A/C usage. However, recently I drove from New York to Dallas, Tx. Water is leaking from coolant heavily every time I stopped. According to a mechanics advise, as I was on the way, I kept the coolant cap very loose, poured water in it every two hours. Everyt time I stopped the water was leaking, but no smoke. The engine heat indicator is showing very normal all through. We reached Dallas four days back, and yesterday as I was on the road, suddenly the accelator was down, I pulled the car aside and stopped the engine. Then the engine did not start. I towed it to mechanic, he said ignition coil to be replaced. He did, and charged $350. When I went to pick up, he said, head gasket is blown and estimated $1700. And then he said water pipe leak, but he is going to do this free. What could be te problem? Was it a gasket blow at all? Or is that water pipe leak? I never had water in my oil? And I never had engine starting problem? In fact it was very quick? Do they have to put the ignition coil and charge me $350? Appreciate your kind concern and happy to award $25 award to technician, if I am informed where I could have saved money. Thank you