Best repair option for AC/Furnace

So I called a tech out to my house because since we have started using our AC this summer we have noticed large amounts of water dripping from the furnace onto the floor of our utility room.

The tech told us the a coil is leaking and we have three options:

Replace just the a coil - $1500

Replace the whole air conditioner - $3000 (our air conditioner is currently ~10yrs old and the tech claims standard life of an air conditioning unit is 15-20yrs)

Replace the air conditioner and the furnace - $6000 (The tech said the leak from the a coil may have caused mold in the furnace which cannot be cleaned. When we bought the house just over a year ago our inspector warned us the furnace was 15-20yrs old and was going to need replacement in the next few years).

Everything is working fine heat/cool wise so my wife and I aren't sure what option we should do so I am looking for opinions and advice of other like minded frugal people.

Thanks in advance for your help!