It's also good that your subject line of your post actually indicates what is going on and what your question is..........Too many posters have been starting an annoying trend of "click bait" where the title of their thread may be something like...You're not going to believe this!!!!!!!!!!

I have contemplated doing a core sample.For what it’s worth, a few of the contractors that have looked at the property have commented on how good a shape the building is in, and in particularly, how good the CMU joints look. 

But I've also found mention of Section 121 exclusions, which seem like they wouldn't apply in any way shape or form in our situation because we will only owner occupy for 1 year before we get military orders sending us elsewhere again.

But in general, beyond info that may be available online (in Philly we have a wealth of information available online), - you should try to catch up on what is being planned, trends and concerns for your area and you may want to look into paying an expediter to gather info and do some research. 

Too much copper is toxic to plants because it prevents photosynthesis.https://homeguides.sfgate.com/copper-control-algae-97682.htmlThat said you should get the roof inspected.You may get lucky, the roof may be in great shape other than the algae on it ( note there are shingle solvents designed to wash off roofs: don’t bleach ( bleach is ineffective against mold) or power wash.)Once cleaned, install the copper as a preventative measure.Other wise I agree with many of the comments, you might have to bite the bullet on the roof, a roof is a big deal to perspective buyers.

Zillow has fairly reliable comparable rent metrics; your projection for Unit 1 is about 10% higher than the average, but you should be able to get it given the location and planned upgrades.One of the magical things about apartment zoning in DC (this is RA-2) is that it doesn't really regulate the number of units, just bulk (=building size & shape).

Our tenants are in good shape & some are working from home so we don't see any immediate payment problems.

If the hinges are still in good shape and the door is adjusted properly, it may be as simple as putting in longer screws.

for kitchen the cabinets are old but Still in good shape.

It was in very bad shape.