I'm much more interested in getting my erroneous assumptions corrected, and learning where I'm missing the mark. 

Would I just send the buyer an Assignment of Contract with all the terms that he and I agreed to, and have him pay me a flat fee up-front or maybe when the assumption process is complete?

The foundation repair company salespeople (and even owners, in some case) of structure companies are not engineers and though they may be right most of the time, there will be gaps in their assumptions.

That initially makes me question your ARV, Rent, and Renovation assumptions.

Thanks for the tip on the assumptions for vacancy.Mortgage Balance: $384k - $76,800 (20% down) = $307,200Rate: 3.875%Term - 30-year fixedAny other possible tweaks to reflect the real world?

I agree with your assumption of tax liabilities as his liability would be extremely low since he has no other income source. 

You also need to look at the comps as well to validate your assumptions. 

Your assumption of 2% and 5% is pessimistic.I assume that these rents ($2,333/dr) are actual rents?

Obviously every strategy has risks and I am making some assumptions/predictions when I purchase a property and lock in for 30 years.

Or am I incorrect with this assumption?