, things to add into calculations (i.e. duplex you cover sewer/water vs. single-family is on tenant, any other considerations?)

Taxes 5500/yr, insurance 5500/yr, water 2500/yr.

Some utilities stay with the house (eg water if the tenant doesn't pay, you get stuck with the bill), others go with the tenant (eg electricity, phone). 

The faint water stain that becomes a ceiling leak?

Back in 2008 and 2009, during the foreclosure wave, I saw cases where people poured concrete down drains, stripped copper wiring from the walls, left water running to flood properties, and caused extreme mold damage.

Beaches, water and view is better than Belize.  

ATUs are useful in environmentally sensitive areas or locations that are less suitable for conventional or gravity flow septic systems, such as inappropriate soil conditions where the water table is too high to allow the drain field to operate effectively.It seems likely your property has appreciated more than inflation and selling costs.  

Start small and test the waters.

After every big storm on Long Island, I see the same issue—homes that look dry but aren’t. A recent case showed 78% indoor humidity and wall studs at 25% moisture content. Six days of controlled drying (dehumidifiers ...

Okaloosa island (fort walton beach beachside) is most likely condos on the water.