The house should only be a tear down (termites which have destroyed the garage and interior of the home, no central air/heat throughout although it's listed as such, broken showers and dish washer, swimming pool with cracks in it, a leaky roof, etc..). 

It’s can’t be the heat.  

Opting for multi-zone mini-split systems (link is just a sample, there are many to choose from) offers a practical and efficient solution for heating and cooling an entire house, particularly in renovation projects where space for ductwork is constrained or entirely absent.

If you have the other expenses, such as the property taxes, water and sewer bills, heating bills if the landlord is responsible for those, electric bills if there is a landlord account, and any other bills you as the landlord would be responsible for monthly you can total them up and see if they will be more or less than the $900/month rent your apartment will generate once you eventually move out of it.

I an planning to split the sewer, water and heat.

They are resistant to scratches, heat, and stains, making them an ideal choice for rental properties.

The heat combined with the intense sun (UV damage) would deteriorate and degenerate the wood, rendering it not at the same level that it was originally [Wood products are rated/graded, you will see this stamped on wood when you buy it and it is called out by architects and engineers]So unfortunately no, I seriously doubt that the wood is going to be approved.

The mold that was there was sort of faint - white fuzzy stuff from the house not being heated or air conditioned during it's time being for sale. 

Imagine not just a pool in the winter people want it heated.

Your rental has to have a new heating system - that’s $10k being drawn from that HELOC that has no direct increase in cash flow to cover the debt payment.