Poor insulation in the wall at that area causing condensation or possibly a leaky drain or supply connection. 

A full bathroom will be added in the finished basement, as well as a new kitchen, upstairs bath, updated electric, and adding a condenser to the existing forced-air system.

@Peter Morgan Residential HVAC systems have two components (hence "split"): an indoor unit (evaporator coil) and an outdoor unit (condenser coil).

By your description might also be condensation VS an actual leak. 

Either way, I know of someone who had a new unit installed and inside the condenser was a cracked line where the Freon was leaking out.  

You only need the square footage, height of the ceilings and look on the furnace and condenser for the capacity in 'tons' and not btu's.

Also, due to vacant house sitting so long, we had our condenser, line set, and detached garage's electrical wiring stolen. 

If it's a heat pump, you'll need adequate power to both the air handler and the outside condenser.

Recently the condensers failed on two of the 5 units at the same time.

____Repair cost break down___Roof - repair price depends on home SQFT, type of roof + any know damageHVAC - replacing a Condenser (outside unit), handler (inside unit), ducts or possibly replacing all together Electric - cost depends on the job - replacing panel, running new wires, adding or repairing electric to an areaHot water tank - cost of tank + labor + ( some plumbing components)Windows - cost per window + labor cost per/windowPaint- cost is price per/sqft, varies if painter are painting trim, primer, how many coats, interior or exterior Sheet rock work - how many sheets 4x8 sheets will they Demo and replace Flooring - normally charge per/sqft - find the cost of the floor you want and multiple by your sqft and add the labor cost per/sqftkitchen - cabinets ( price per cabinet + labor to build and install) - counter top cost, price for material sqft and price per sqft of to fabricate, cut, and installAppliances - cost of appliance + install Bathroom remodel - depends on the size and quality finish - i start with finding the price for a small standard 5x7 bathroom and move upPlumbing - cost depends on the size of the job and scope of workThe above information is a quick break down of how to get rough estimate of rehab cost.