If your windows are old and are showing corrosion, that bill falls on you.

Nicotine smoke is very corrosive and they fail much quicker.

Old diverter cylinders are fraught with corrosion & can be tough to replace after the fact & you can damage the diverter removing it.

According to the internet, "Pipe lining is a way to restore the pipe from corrosion, leaks, or cracks from roots.

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I always check the panel & look for corrosion, defective breakers, outdated defective panel models & ungrounded circuits evident within the panel wiring.If the old meter is a 60 amp you're in for upgrading the service wire, weatherhead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.