Inspectors for two of the properties told the buyers that new water heaters were necessary only because there was  very little corrosion on the water pipe nipples on top of the water heaters.Just last week, one inspector made be get mold testing because he said a black mark on the door where the door was closing against a 2 inch x 1-1/3 inch portion of the door appeared to be mold.

Galvanized lines should be replaced - if they are not already constricted to a trickle due to internal corrosion, they will be.  

It looks rusted right where it comes out of the ground in the crawl space but it hard to know how much corrosion inside - it's probably the original 100 yr old pipe, so I'm guessing replace as well?

this is the scope of work. the dimension of the roof is roughly 45X151) Tear-off all layers of roofing and gravel down to wood decking and legally dispose. 2) Inspect wood decking and replace all deteriorated and broken areas at $ 2.50 per sq. ft. 3) Inspect existing Ceramic Copings and replace all deteriorated and broken at $ 35.00 Each.4) Install Fiber Cant Strip at the base of the walls and curbs.5) Install 1 layer of Base Sheet over wood decking using corrosion resistant screws and plats. 6) Install 1 layer of Modified Bitumen CertainTeed GTA Granulated. 7) Install Metal gravelstop and gutter flashings at the roof edges. 8) Flash all vents, soilpipes, and chimney using the same Modified Bitumen. 9) Install 5” Galvanized Box Gutter and 3x4 Downspout to replace existing. 10) Pick up and haul away all roof debris generated by Presidential.

@Babatunde Onadele JrI would guess the winters and the amount of snow would have an impact.The lake is not salt water so you do not have the corrosion you have near oceans

Copper can not react well to the corrosiveness of concrete. 

This can lead to additional corrosion w/in the pipes, possibly leading to premature failure of the heat exchanger, which is not inexpensive to fix as I just fixed one on one of my properties.Assuming you're on gas, you could (imo: should) install a condensing gas boiler.

Galvanic corrosion occurs wherever two different metals touch.

But did they detect corrosion and damage in cases like this?

The condensate drain and trap need to be cleaned, and the water replaced in the trap.The blower wheel needs to be checked for debris and cleaned if necessary (this requires complete removal of the blower wheel).An amp-draw test should be conducted on the blower motor and compared with what is listed.The wiring needs to be checked for corrosion and damage.The filters need to be checked