How can I design or propose these apartments to meet this requirement while making them functional for long-term residents?

When I hear of historical significance, I think of an old building with significant cap ex, and functional obsolescence.

I also converted the other side of the property (it's a functioning duplex) into an Airbnb / Short Term Rental.

Not a major rehab - flooring, paint, and some misc. work  - not a full blown renovation.PS: I wish BP could really have a search function because the current one sucks. 

@Timothy Fortin As Michael's superb content here and elsewhere emphasize, cost segregation services are tightly intertwined with broader tax functions typically provided by accountants.

The city approved rental license inspector essentially just tested to make sure there was access to hot water, access to heat, no unsafe conditions like exposed electric wiring, CO/smoke detectors present/functional, fire extinguishers present/not expired, etc.

Doesn't Buildium have an accounting function though?

The difference in being able to apply more money to the HELOC functions exactly like making a big chunk payment to a mortgage, which significantly reduces the interest accrued over time. 

Once you have a shortlist, sign up for an account with each one and test them extensively to see how they function.

It' a function of price now, and if you can find a deal that is priced accordingly, then you buy it, especially if you like the fundamentals long term