If I'm buying old then that it becomes very important to understand capital expenses for upgrading plumbing, electrical, fixing foundations, etc and understanding the properties current condition. 

Or a SFH that has a detached garage or work shed and convert it (running electrical and sewage might be expensive so do your due diligence).

Marble backsplashes, all new kitchens, bathrooms, HVAC, electrical, refinished hardwood flooring, etc, all with the idea that we would get better tenants, higher rents, lower vacancies, and less maintenance/repair requests. 

Having 1-2 go-to vendors for the highest frequency problems… plumbing, A/C, septic, electrical, and roofing, I can pretty much make a 2-3 minute phone call and pass the problem to someone who will solve it with little more input required from me.  

Peeling paint, inside or outside, handrails on stairs over 2 steps, broken glass, working electric and plumbing and operational heat source. 

EV and Green energy!

You'll be better off spending your energy learning how to hire the right PM instead of trying to PM it yourself.

They all have their own separate oil tanks/electric meters etc. and there is well water.

We added a door that we deadbolt to separate our side from the airbnb side, and I had to have some electric outlets and an overhead light to the airbnb side, but other than that everything was updated already and we just had to furnish the property which was really nice and made the process pretty easy. 

@David De La Torre it is common for electric and gas to be turned off when a tenant moves in, forcing them to turn it on in their name.