Median household income can be an indication of the area.It's not feasible to manage D-class remotely or with 3rd party management and make a profit.

However, I do not know who this could be beyond a property manager which would not work in my case since I am looking to self manage for both tax (material participation in STR loophole) and profitability reasons. 

I am guessing the beach houses were usually purchased and rented with a goal of “free” family vacations being paid for (along with mortgages) by rental income.  

Can anyone recommend software which would accept monthly payments (from me to seller) and create proper 1099 forms for tax purposes at year end? 

STR management is 20-25% (I would recommend self managing or getting a VA), since income is higher you could probably get away with 5% for maintenance / cap ex but remember you need to cover utilities and cleaning for these units which drives up costs.

I owner occupy the home, renting a room to a friend at $650/mo, and the lower unit rents for $1850/mo - total rental income of $2500/mo.

For example, I mention that every adult must pay a $30 application fee, pass my credit/criminal background, make a combined income of 3x the rent, and whether or not the property accepts pets.

Not going to kill it on the rental income, but if you sell it subject to, there is a lot of equity.

The projected rental income wouldn't cover the mortgage and HOA at this time.

Any way around paying the very high property tax, school tax etc. in SC?