How many hours "will" it take to replace a heater

My sons 1995 pontiac grand am 4 cyl with 119K miles- overheated and leaked anti freeze on the ground. His airconditioning has not worked all summer, but he did have good heat last winter. He poured water into the radiator and it just flowed out again. We had it towed to a repair shop for diagnostics. He was told that it would be about $1000 to fix or about $250 to repair. $600 for a new heater core plus labor. Or for $250 they could bypass the heater core and make it run. I may be a woman but I"m not dumb enough to just say go ahead, so I have been checking on prices for parts- checker auto has new parts for heater core and radiator and thermostat. $200 for parts including a radiator, thermostat and heater core. After talking more with my son, he said the mechanic also said it could be a head gasket. Which I believe would be about $1000 to replace. Why if it has a bad heater core and or bad head gasket would the water leak immediately out of the area right behind the right front tire. I know absolutely nothing about vehicles, repairs etc. So any help would be appreciated.