The last step was getting the oil  furnace repaired which is responsible for heating the hot water (coil).  

It's not at all difficult from a technical point of view, nothing that can't be learned well enough by practically anyone by watching a few YouTube videos.Additional materials costs are maybe $10 for the rockwool insulation to insulate old window weight pockets, a can of Great Stuff Window and Door foam ($6), and a 10 oz. tube of silicone caulk ($6).This is pretty much what the window replacement companies will charge you $550 and up for, with the addition of what's called window wrapping from the exterior, leaving the exterior trim of your window with vinyl covered aluminum trim coil pieces that they tell you will make your windows "maintenance free."

The compressor on an old (freon) AC unit is locked up and the coils are rusted.

@Seth Coil-Depending on where you live, double check that the taxes are exactly what is displayed.

The reversing valve changes the flow of refrigerant making the outside coil the evaporator and the inside unit the condenser coil.

First the coils all need to be cleaned both inside and out. 

Apparently, the coil had been replaced in 3 of them. 

In addition to the items you mentioned, I have the HVAC serviced every spring including cleaning the condenser coils, lubricating the motors as needed and checking the refrigerant levels.

Vinagear scrub the walls and ceilingsOil based Kilz paintClean the flooring with vinagear Ozone machine when not working in the propertyClean out the duct workReplace anything that is fabricReplace AC filters and run constantly and then replace againScrub the fridge coils and any surfacePaint it againClean it againRepeat process as needed (it sucks and it takes someone moving in and absorbing the rest of the smell)Whatever you do don’t turn of the electricity to the unit.  

The “issue” is pipe sizes and a leak in the evaporater coil.