Now the details, price, condition etc.. 

Sellers who are in financial distress or dealing with a property in poor condition might be more open to this type of arrangement if it helps them get back on their feet.The key to making this work is transparency and clear communication.

Quote from @Matthew Cook: Dig into those comps with a realtor doing business in that market.

Congratulations on jumping into the STR market. 

This provides an international/affluent tourist pool whom are extremely unlikely to disappear regardless of local economic conditions.

If you are willing to do this over a long period of time, this can be a great strategy, especially in a low-inventory market.

I have purchased an off-market property from an investor who now wants to sell the rest of his portfolio.

It also depend on the condition of the property. 

Remaining Balance: After the balloon payment, the remaining loan balance will be re-amortized over 5 years (60 months) at the same 6.5% interest rate.Payment TermsInitial Loan Terms (First 24 Months):The monthly payment for the first 24 months is based on a 12-year amortization schedule:Monthly Payment for First 24 Months: $1,573.39Remaining Balance at Month 24 (Prior to Balloon Payment):The balance after 24 months can be calculated using the amortization formula:Remaining Balance at Month 24: $157,113.92Balloon Payment:The balloon payment will be 50% of the remaining balance at Month 24:Balloon Payment Due at Month 24: $78,556.96Re-Amortization of Remaining Balance (After Balloon Payment):The remaining balance after the balloon payment will be:This amount will be re-amortized over 5 years (60 months) at the same 6.5% interest rate.Monthly Payment for Final 60 Months: $1,541.18Summary of Key Terms:• Purchase Price: $215,000• Down Payment: $43,000• Loan Amount: $172,000• Interest Rate: 6.5% (fixed)Initial Terms (First 24 Months):• Monthly Payment: $1,573.39• Balloon Payment Due at Month 24: $78,556.96Re-Amortized Terms (Final 60 Months):• Remaining Balance: $78,556.96• Monthly Payment: $1,541.18Additional Conditions:1.

I have looked at other coastal markets that are less expensive than Charleston proper.