The problem we are having is that the air coming from the downstairs vents smell "mildewy" but not all the time.

When I walked through, the house no longer has any of the plumbing (galvanized steel) and none of the old wiring knob and tube, and actually is a blank canvas inside still would need to be built back from the studs to a five unit.What am I missing?

If you look on Air BnB you'll see all of them.

I find it so petty that media, society, etc fails to notice real issues unless an Elon musk video announcement is aired to start conversations like this.

Any Knob & Tube or Aluminum Wiring3.

There is a qualified intermediary but left some of this up in the air, Bill Exeter helped me out and answered the question. 

My husband is retired Air Force and he used the SkillBridge program to work with an realtor/investor out of CA.

What do you guys think this house could sell for fully renovated(New kitchen/baths/floor/lighting/new central air unit)? 

Eventually our long term goals would be to purchase properties on some of Maine's lakes to Air BNB them.

Town water and sewer, forced hot air heat.