In addition, doing fewer deals than that will most likely mean that they don't have a legit team of pros to back you- PM, GC, Plumber, lender, etc- if they aren't feeding them constant business, those contractors won't drop their current jobs to help you in an emergency. 

If you decide to go this route, you could inform the tenants that they will be responsible for a separate monthly utilities charge in addition to their rent.

Do they charge any fees in addition to their asking price for properties?

Additionally, the price was initially listed at $795,000 and is now down to $645,000.

What do you mean by the additional $18/sq design fee was not included? 

Also make sure your GC and all subs list you as additional insured on their policies.

I actually have two options for this property: 1) Rent out the existing 25 storage units AND rent out the building plus add the 25 additional units out of pocket.OR 2) Rent out the existing 25 storage units and added the 25 additional units out of pocket to rent them out BUT keep the metal building for my GC business to where we have offices and storage for building materials.

ADUs additions in single family zoned areas add less value than the hands off cost of the ADU addition.  

Do you intend to purchase additional properties?

Here's an article with additional FAQs on cost segregation studies that you may find helpful.