With home air conditioning it's mostly about temperature difference when you, the homeowner, are investigating whether or not your system needs attention.
Measure the temperature being pulled in by the return air grills in your home. Next measure the discharge air temperature out of the closest register to the furnace. Subtract the discharge from the return to get what's called Delta T or temperature difference.
Depending on the age, total system set up and other variables you should see a difference somewhere in the range of 16-20°F of Delta T.
A properly installed residential central air conditioner is a permanately sealed system. For the life of the equipment there should never be (knock on wood) any reason for moisture to get in or a reason other some types of failures to ever need to take the charge out and replace it.
The recommendation of a new furnace could be that the tech found the maximum air flow capacity of the old furnace doesn't match the requirements of the new AC.
Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.