For reference, I had a renovation that we redid ALL plumbing from under foundation to all new kitchen and bath layouts.  

If all the plumbing was shot, that should have come up. 

You don't have to worry about plumbing, bad tenants, fixing issues. 2.

Install central air and the next thing this guy is likely to complain about is his high electric bill.Keep in mind that in many states a landlord must provide reliable and working plumbing, electrical and heat. 

Do you have any recommendations for contractors for roofing or plumbing/hvac?

These are 1930 buildings with old plumbing and electrical systems.

That's at least 10 bathrooms, 10 kitchens with all new plumbing and electrical. probably new hvac too.

Permits will also be required for your plumbing and electrical work.So, what if they catch you?

So I’m in a bit of a bind, I have a duplex I’m house hacking we remodeled entire thing, new hvac, electric panels, electric main, updated plumbing, water heaters, furnace, install central air, roof.

So, given you are in a lower end area with lower rents, you need to reserve more money on repairs, maintenance and capex compared to a higher rent area, since that $200 HVAC or plumbing service call is 25% of $800/mo, but 10% of $2,000/mo.