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.

Corrosion, rot, termite damage, age, are not Tenant responsibility.

Also you must replace any plugs and switches with aluminum compatible units or the alternative is to use pigtails that join the aluminum wires together and branch off with a copper wire to the plug or switch.In all cases every connection should have a compound called Nolex which is a paste that protects all connections from corrosion making them safe.

There's a technical reason why that happens, which is due to corrosion, but you can't tell from the outside of the unit. 

It also indicates whether their water is corrosive to lead and copper also.

They tend to be corrosive to wiring/pipes/etc and reported to cause respiratory problems.

Corrosion in outside older breaker box can contribute.