There are few things more corrosive than the belief on the part of your tenants that you are getting rich off of their rents.

The property is about 70 years old at this point.The water heaters, while not very old, already have rust/corrosion on the top and bottom of the tanks. 

I always check the panel & look for corrosion, defective breakers, outdated defective panel models & ungrounded circuits evident within the panel wiring.If the old meter is a 60 amp you're in for upgrading the service wire, weatherhead etc Then some major upgrading within, especially installing code compliant dedicated circuits.

Why , rat droppings and cat urine are unsanitary . have all the windows and spray it down , be carefull you need good ventilation , dont spray on electrical outlets , it is a corrosive .  

There are a lot of minerals in the water and they settle at the bottom of the tank.There are anodes in tanks that take on the brunt of the internal corrosion, but when those go the tank will take on the rusting process and eventually go.

I do not know if there is any long term maintenance plan other than keeping an eye out for corrosion, leaks and potential for pipes freezing.

Plus, it's not a pH neutral product so you are potentially adding something a bit corrosive to your discharge plumbing. 

As I understand it, remediation requires completely gutting the house down to the studs and replacing all of the drywall (pretty much every wall and ceiling), in addition to replacing any electrical wiring and components with corrosion caused by the drywall.

You may need to closely inspect all the switches and receptacles for corrosion.

Note: Salt is super corrosive.