The age of the HVAC should have been disclosed or obvious when you walked through (ie you could see it was older). 

Hello BP community,I am a newbie landlord and I have rented one of my units (SFH with 3 bedrooms/2 baths) to an older couple and their adult daughter.

Maybe sell the one or two that are older, worse area etc. you could use the money to pay off your personal or the loans on the other two.

BiggerPockets is full of resources designed to help you along your journey, whether you are just getting started or a seasoned investor.

We used to have in-house inspectors but it was hard to scale and with our heavies season being that of summer months, at 1000+ units we weren't able to hire enough inspectors for the last day of month move outs and first day of month move ins and then still keep them fully-employed in our slower season in fall/winter.

I do have an empty lot next to my current SFH , doing nothing (but costing me money to mow :)) my thought was to build a detached guest cottage to serve as an overflow for guests or it could also serve as a seasonal / monthly rental.

BiggerPockets is full of resources designed to help you along your journey, whether you are just getting started or a seasoned investor.

The alternative is to find a seasoned investor in the location where you plan to invest, work out an agreement with them to bring their money, time, expertise, relational capital, systems, etc. to a deal you bring to them, and let them show you the process from beginning to end.

Occasionally, like maybe twice a season a guest would go out and read in the lounge chairs on the deck.

Currently, I am a seasoned Contractor / Residential Builder working for clients.