I would strongly recommend you get linked up with an accountant who is more real estate focused in order to avoid any further costly mistakes. 

I don't think it should cost that much to build.

We will provide comfortable homes for renters at a reasonable cost.

Adding AC doesn't cost much and adds a lot of value to the home, so I recommend installing it.

The property features two 1,400 sq ft suites on the main level (one currently leased) and over 3,000 sq ft on the upper level, with 750 sq ft already occupied, 3 small office spaces (+/- 250 sq ft each), and a basement tenant with 1,400 sq ft of storage space.The building generates $6,000 in monthly gross income from its five tenants, but with full occupancy and market-rate rents, it has the potential to bring in $10,000+ per month.Given its historical significance, stunning architectural details, and location, I’m curious about your opinion: Would this property be more marketable for sale with or without the current tenants?

I suspect there’s also a correlation between the lower eviction rate operators and longevity of operation, longevity of leases, reduced management/operational costs & greater appreciation.  

However, I've recently come across articles discussing the potential benefits of lower down payments in certain situations.Given the current real estate market conditions, what are the critical factors to consider when deciding between a high or low down payment for an investment property?

They will cover all costs, purchase and rehab, here after.

I use to think PM cost 10% of gross income.

I can't speak for all investors, but I know I have become more conservative with the rising cost of construction, interest rates, insurance costs, and vacancy hold outs.