In this instance excessive corrosion was found and all piping was removed and replaced with PVC.

I have three other units that are already showing signs of corrosion on the bottom seam, so that's 33% of my water heaters that don't even seem like they'll make it 5 years! 

I have seen many fail due to age, either due to severe corrosion of the housing, or, ultimately, the windings of the armature became unwound, due to internal wear.

Also as a landlord you're in the customer service & sales business.Watch out for properties with galvanized steel plumbing since depending on the age corrosion can become a real problem and affect your water pressure.

This scaling also leads to splitting of the pipe due to the expansion of the corrosive material.

If the water's that hard, you're going to be having routine trouble with mineral buildup and corrosion in all your plumbing that you're going to have to be on top of.

Look for corrosion on the joints of copper water tubing to determine excessive flux use?

This can be costly, particularly if the property is located in a corrosive, beachfront environment.Regulations: Some beach areas may have regulations that limit or prohibit short-term rentals.

Corrosion was everywhere inside the boiler, which none of the techs noticed until the initial installer came out to investigate. 

Aloha,Based on that age, you are due for a lot of work...waste and sewer lines, possibly supply lines ("some plumbing"is worrisome); potentially electrical panels and wiring if there has been ongoing water intrusion causing heavy corrosion of conduits and panels; roof; internal structure (termites, wood rot) exterior paint/siding; along with routine service call items.