Plumbing and heating was first, good choice.

2) Tenants might argue against landlord due to hvac heating, gas leak etc fixes, say it is landlord negligence etc etc.

What are your heating options? 

And more recently I had to pay 900 bucks to have someone trouble shoot a boiler/ heating issue at my rental ( the whole heating system was new).

Is it a boiler supplying hot water, or a boiler for heat?

These unfortunate tenants had to go through crap like going without heat for a month in January, and having a water leak going unrepaired for months, with a make-shift sump pump in the basement keeping the flood at bay (the story was the PM died, but judging by the quality of the "repairs", they had more issues than that).

We also had my personal Heat and Air team come take a look at the system to make sure it was fully functioning and without any leaks.

We also had my personal Heat and Air team come take a look at the system to make sure it was fully functioning and without any leaks.

Took out the electric heating and put in a forced hot air/cooling system.

However, markings on the heat unit were incorrect - turned out the York Furnace was actually an oil heater.