Then sell the property and start doing fix and flip or fix and rent. ( I prefer to sell this property as it 100km away from my student rentals. ) Not sure if I should renovate the lower unit to sell.

Then you subtract your expenses: (1) Fixed cost: property tax, STR insurance, utilities, landscaping, etc

I.e. say your plumber has a fixed $150 service call fee.  

An expensive fix that could've definitely been avoided

Fix the problem and make them pay for it.   

Sometimes they want me to fix something (like cracks in a driveway that no other company had cared about previously).  

using this form of financing is a great way to acquire/fix a property but if you are looking to hold long term you would want to find long term financing. 

As far as selling... you will have to disclose the problem whether you fixed it or not.

Expensive unit at the property goes out, takes money to fix.

Investment Info:Single-family residence fix & flip investment. 176 Wildwood Road is a single-family home located in Sag Harbor, NY 11963.