Broken Air Condition 70 year old mom

Hello everyone, thank you for condsidering to help our family. My mother is in need of a new air conditioner and to increase better air flow and function, add 2 more air ducts and 2 more return vents. The house is a modest 1700 square feet build in 1987 with one return vent. Unfortunately living in Georgia is very hot and humid summers. The outside temp is 110 and inside is 90 degrees despite setting the thermostat to 76 degrees. We have been told it is poor construction compared to today's function. For now, there is a window unit in the living room, ceiling fans, and stand up fans throughout the house. She refuses to come stay with me during August, our hottest month in Georgia. I received a quote for 18,000.00 for installation, equipment, patch work to walls and/or ceilings. We are going to repaint the areas ourselves. Any help is greatly appreciated. May God Bless You for all eternity.