The hardest part would be if the old drain wouldn't unthread from the shoe below due to corrosion but I was able to get it out without too much elbow grease.

Most areas have hard water and the scale will build up in the hot heater as the water evaporates from the system thus causing corrosion to the hot heater elements.

Look for mold issues and corrosion.Fire/smoke can be corrosive, so it can have a huge impact on anything that is metal within the structure.  

Galvanized steel, if used, could cause you a big headache in the long run due to rusting/corrosion issues. 

Soot is corrosive, it is possible that almost anything metal will become pitted and need to be replaced.  

Age or corrosion are not damage.

as the home was built in 1923)-Slight cracking on the foundation on a part of the house (the pre-inspection noted it is common in older homes and not a big concern)-the plumbing had signs of corrosion which is why the inspector made the note of leaks at some time.

Plumbing by-passes or abandoned lines; collapsing sewer lines; badly corroded supply and waste lines (both can be from inside or outside corrosion) fixtures with very hard to find components; hidden electrical junction boxes...or non-existent boxes at junctions; outdated and/or underrated distribution panels; knob and tube wiring improperly connected to, or extended with, romex; Switches wired incorrectly, breaking the neutral instead of the hot side; no, or poor grounds for improper outlets; overheated connections, outlets, switches, and light fixtures; improper wire sizes for circuits and overloaded circuits; and plenty of others...all very costly to troubleshoot, locate, and correct individually.

See manufacturer’s specifications.)7.Location of Fasteners: Corrosion resistant fasteners shall be located per manufacturer’s instructions and not placed in factory applied adhesive.

This could be zinc oxide from galvanized pipes corrosion.