normally there would be a balloon payment set up for some time period.  

Other options such as seller financing, hard money loans and commercial loans that I have looked into all seem to require a balloon payment of sorts which I am not convinced I'd be able to refinance it for the balloon payment being that it has shown difficult to get a loan on it as it is now. 

I would probably try to speak with them face-to-face or on the phone and finish the conversation by telling them you will send them a formal letter shortly with the exact date after you talk with your contractor/partner, etc. 

Therefore, I would recommend sticking with a marketing channel that allows you to reach more people and convert more efficiently.I believed that direct mail could be a good marketing channel for rural areas, where people would appreciate a letter approach more, as they are not bombarded with SMS and calls.Feel free to reach out with any questions!

And get a detailed report on their letter-head. 

My worry is the 5 years pass and the balloon payment comes due, and now I am stuck with whatever interest rate is available at the time.

I’m in the process of signing an offer letter for a home in the Lake Carolina area of Columbia, SC, listed at $399K.

Also I feel like I could offer more with seller financing, but confused on how to come up with that amount of $$$ for the rehab, pay off the balloon AND construction loan.

When I'm preparing these letters/documents, I'm really learning as I go.

I would then follow that up with a lease violation letter aka notice to correct.