The house had some old cast iron piping and older electrical panels and distribution.

So the tiny home do not have electricity or water.

Looks like your missing water/trash, bookkeeping and some electric (basement and hallways).

Yesterday I met the new tenants at their future home to hand them the keys and go over the operating basics for their home, gas, water, electric, filters etc.

We will add central air and new furnace, and electrical panel in 2024 or 2025...so some upgrades needed, none urgent.

Of course the work has to be done according  to current building codes, however, the same scenario in many places in the states (specifically in California) is completely different, because you have to schedule a city inspection for each construction milestone (rought electrical, rough plumbing ing, sheetrock, stucco and so on) and each inspection could take  4-6 weeks before the inspector is available to come to the site (at least that is the case in the bay area).

I did most of the work myself except for using a few specialty subs i.e. electrical, plumbing,HVAC,etc.

I suggest place 1000 volts electricity charge trap so when he goes for it ZAPPPP like a mosquito.

I know its a new building but after you factor in taxes, insurance, management, repairs, accounting/legal, electric  gas, water, trash services the cash flow isn't enough to make it worth it. 

Would I need to convert one of the units to electric heat (I'd rather not since electric heat is a lot more expensive)?