Be aware of the costs as you'd need plumbing (water and sewer), electric and possibly a separate meter to the new unit.

And you have a vacant unit to do the "proper" plumbing repair.If you offer them a rent concession, my money is on them going back to that well each time there is a problem, asking for yet another abatement. 

Many carriers have ceased offering new policies or will only accept properties in very nice condition with verifiable updates to the roof, electric, hvac, and plumbing.

I would say look at the big ticket items, roof, foundation, electric, plumbing, HVAC and assess from there if anything needs to be done 

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.