Currently, there is only 1 huge old gas hot water furnace that services the whole house and 2 hot water boilers, 1 services the basement and first floor and the other one services the top floor.I have been quoted $18k for a complete forced air, central air system for the 3 units.

Most rentals i see -- the rods are broken often....and are using the spring loaded replacements

I live in Seminole County, FL (Winter Springs). 

Siding, Windows & doors, trim paint) Interiors typically work from the basement up in this sequence - DemoMEP- Mechanical Electrical Plumbing rough in Insulation and air sealing Drywall hanging / taping and finishing Paint or wall finishes Finish flooring / tile, hardwoodsCabinetry / casework / wall tile in kitchens and baths Interiors doors / baseboard / crown molding / door and window casing Finish plumbing and electrical - also called trim by tradespeople That's it in a nutshell, some may use different sequences but this is a general activity framework.

The central air unit/coil was newly installed a few months before I bought the place so I'm not expecting/planning any capital expenditures on the unit. 

Same with air units.

My brokerage's research department puts out a monthly market report for Southwest Florida including Cape Coral, Fort Myers, Fort Myers Beach, Sanibel and Captiva Islands, Bonita Springs / Estero, Naples and Marco Island.

(Of course, that’s not in writing) He said he could feel the cold air coming out, when he was on the roof, when that cold air should have been coming into the house.

Don't put a house on the market in the off season :-)If you do, don't despair - it'll sell as soon as spring pokes its head out.

They have central air plus two portable units we supplied.