The addition of a private balcony and standout interior design further elevated its appeal to both short-term and long-term tenants.

Also, if I found a 4/4 in the right area for the right price, I would still want to close up any living and/or dining areas and any interior community areas.

We have added new interior features and a new roof.

You will likely have common area lighting (interior and exterior), perhaps heating and cooling too. 

But I recently upgraded and used a local designer who has given me full sets of interior colors and exterior colors so we can properly design each home

I have a background in interior design and have a real estate company.

In addition, several items can be done simultaneously (I highly recommend this to save time as time is your enemy) such as doing the exterior siding or stucco at the same time interior rough electrical and plumbing is going on.Foundation is always first and rough framing follows that (which includes windows and exterior doors).

These updates include modernizing the interiors and improving the overall appeal of the building to attract tenants.

I am wondering if the interior stairs are typically removed to allow for more sq ft in the basement unit as well as greater security  between the two units?

I do not recommend backyard especially if you have a pool or hot tub and certainly nothing interior!