The "newest" property I have ever acquired was built in 1961.Things to look out for, but are all fairly obvious during inspection: type of wiring, boiler vs forced air heat/air, foundation issues, tuck pointing on a brick property, sewer line scope (particularly if trees are near front of property).  

The highest rated windows were 1/4" glass panes on both sides in a dual pane set up, and then they began to use laminated glass, thicker air gaps and eventually triple panes.The other thing is cost.

You say there is no Central Heat and Air in a 4 Plex?

When you list a property on air Bnb through a management company, does it show them as the host or you?

Apartment residents noted a need for more storage space (31%); a dishwasher (16%); better Wifi (16%); parking (15%); central heating and air conditioning (11%); and in-unit laundry (11%).Renters are willing to pay more for these amenities.

Am I able to rent my Condo out on Air Bnb for the 40% of the year that I will not be living there?

I'm still up in the air but my above statements are my current plan.

To provide some perspective, I'm currently AD Air Force... and I remember commissioning back more than just a few years ago. :).  

Nowdays people can buy portable AC and heaters or I can provide them so there are only a few situations then property can be uninhabitable.

Air Touch HVAC.