Add on potential electrical main panel upgrades to handle the increased load (assuming no gas service, they'll be cooking and heating with electricity in the ADU), along with drawing and permit fees and you're maybe another $5k. 

Now they said ok to drop 25k and we cover for everything including the hazardous flags.

If I didn't need the cash to buy our next home I wouldn't dream of selling it because of the great rental potential.

It was expensive upfront but it has the potential to be developed.

I'm interested in potentially moving East, I currently live with my parents in a small town in Ontario.

Look at the whole market and all the factors that would effect income potential. 

Just funded an acquisition there that had a year of successful booking history, and the owner thought had even more potential.

Personally I think I don't want to deal with pets because a) it's a condo complex and I would feel bad for the neighbors if a dog was sitting in the unit barking all day while the family was at the beach, and b)  I live 900 miles away and would not be available to assess any potential damages incurred by someone's pet.  

Most of the specialty cost seg providers will do an upfront analysis and give you a range of the potential deduction and take into account your federal and state tax bracket to determine the tax benefit.  

So, on a turnkey 5+ unit property you could find ways to still potentially add value and do something like a BRRRR potentially but mainly because these properties are considered "commercial" and valued using the "income approach" when an appraisal is done.