Be "car-ful", you are in big-bucks territory. My friend's Jaguar leaked into the passenger carpet, smelling so bad,,, it was parked. Repair was estimated over $2500. Shade-tree mechanic simply coupled the heater in&out hoses together. This eliminated both the hot-water-heater, and the problem. Ah well, the AC works and we live in Arizona, and he still drives it to work.
Your mechanic pressurized the entire cooling system and found only one leak? Generally, a heater hose that old will 'weep' at the end fittings and corrode any metal it finds. Pressurize it again, and find ALL the leaks. Pay extra attention inside the car, when it is at normal engine temperature, run the heater full-blast with the windows rolled up. If you can smell the leak, and the windows get steamy, you have a serious problem with the heater 'core'. If there is no issues inside the cabin, then replace all heater hoses, and the heater control valve.
Hope this helps.