A duplex in Miami will probably double in value in 10 years (and so will the rents). 

I am primarily buy and hold and look for value, and opportunity for STR.

By putting 10% down I probably will be less than breaking even every month ((like an extra $800-900 per month at very low/conservative rent prices)  with the building fully tenanted (as is condition not counting value add with renovations). 

Assuming it is a residential property, it will be roughly 3.64% per year of the actual house/condo/townhouse/etc value (not land)

As for the basement, it is an inexpensive solution to add value to the property. 

I want a better CAP rate than the market CAP of 8% so dividing $56K by 9%, I get $623K as the value of the property.

With greater appreciation, you can leverage that increased value through a HELOC or cash-out-refinance to purchase or rehab your next investment property.Where to begin?

Try to find out what problems they might have and offer value in a solution!

That's ~12% Cash on Cash return which seems pretty good/decent.My concerns are by doing this, it would reduce my rent price on the main house (most likely since they'd be sharing with other tenants on the property) and also I'm unclear whether adding an ADU would negatively affect the value of the property.Option 2Consider converting the property into a short/medium-term rental, specifically to travel nurses.

Not every existing multi-family is an ideal candidate for condo, but usually it is worthwhile and the sale prices of the individual pieces far exceeds the total un-condo value.