As far as the mold goes  get an inspection .it doesnt matter what the electric bill will be , that can change  drastically depending on the tenants usage and the weather , both you have no control over . 

They have high flush toilets in all their units and are paying the water and sewer for the residents excessive usage and just don't care. 

Also, if you’re househacking, and the property is in an entity, does that mean you will be paying rent to the entity for your usage?

On January 1, 2015, you convert the property into your principal residence, where you live until you sell the home on January 1, 2018, for $350,000The Non-Qualified Usage is 3 years and total ownership is 6 years.

Then make decision to calculate gross rent to cover peak usage rates that way tenant has utilities built into rent and you save expense of all the HVAC  work.

we continue to diligently pay for our usage and fees.Yesterday our water gets shut off. 

I'm getting the numbers from the water/sewer/trash people, but since the cost is usage dependent, I would also like to gather some real world numbers, or rules of thumb, in order to estimate these costs on a monthly per unit basis. 

Water usage and visual inspections during repairs support the tenants claim of just themselves and their own children. 

Old systems are most often oversized, which isn’t good for your energy usage nor the quality of heating/cooling in the house.

It seems to me like the HELOC would be better for short term usage, such as repairs on properties, or flips, but less optimal than a traditional mortgage for long term buy and holds.