I rent out the main house, Air b'n'b the cottage, and live in the barn apt.

You might be better off leaving it as a 2/1 and using that $38K for a down payment on a new rental.the 1500 includes taxes and insurance and this property could make a good air bnb, it has a lake view and is 10 mins from the beach.

Or putting it up on air bnb/VRBO/booking.com as a month minimum stay.

When someone upstairs jumps on the floor, the air in the empty cavity between the floors (inside the ceiling) is going to vibrate and create sound - no carpet is going to stop that.

If not, maybe the problem that caused their need has long ago been "handled".Also age of heating system and type (forced air vs steam vs coal).A property inspector will give you a written report of what is found out of line.Good Luck!

In fact, a Rand Corporation study of active duty Air Force members found that the average tour length (length of time in one location) is 53 months for enlisted Airmen and 34 months for officers.  

@Joaquin JohnsonLook at similar STR properties on Vrbo and Air to see what your cashflow could be with a STR.

I don't even do some of the jobs any more and I can come within 10% even on oddball stuff; one of my rentals we recently replaced the air handler, replaced the outside unit, and relocated the air handler into the attic and replaced some of the ductwork.

Is it worth it to replace it with a forced air heating system?

Is it worth it to replace it with a forced air heating system?