Air Conditioning - Please help me make an informed decision!

Some of you have been helping me already, but here is the full situation. I had a tech out today to evaluate the system. Background: Replaced AC condensor (10 seer) two years ago, it was old, we did not replace the acoil due to clearance issues and cost. At the time we thought that was the problem. This week problems again, not cooling. Tech said it was low on freon and the Acoil was the culprit, however he didnt check for an actual leak, just do to age he felt that was the problem. System was way low on gas. He got me a quote to replace just the A-coil, if they can find one (clearance issue only 18 " above furnace to work with) for $800 dollars. They cant quarantee that it will fit a new furnace down the road. The furnace is approx. 15 years old but I just put almost $300 into fixing it. Do I replace just the Acoil and take my chances on the furnace continuing to pur along or replace both in the middle of summer when its 90 outside?? I can find a way to pay for it but...not sure how long I am going to be in the house. Of course I thought that when I shucked out the mula for the new condensor. Is there a standard recoupe time for new HVAC equipment installs and when you see a return. Sorry for the long post, hope someone can help a girl out. Much appreciated!! Marsha